Message for invalid entry

I would like a message to pop up on my spreadsheet if an entry is either blank or greater than 20. Currently an error message will pop up but it will allow the user to navigate to the next cell without correcting the information. If the user inputs "25", I would like the message to pop up and then the original cell to be selected in order for the user to be forced to put a value between 1 and 20.

My current code:

Dim strCurrentCell As String
    Dim strB3 As String
    Dim strB4 As String
    Dim strMessage As String
    strCurrentCell = ActiveCell.Address
    strB3 = "$B$3"
    strB4 = "$B$4"
    
    'Data entry controls

    strMessage = "Check your entry for Customers," _
        & " there can only be up to twenty" _
        & " customers per class period."
        
    If strCurrentCell = strB3 Then
        'Assignment of Values from Work Sheet Variable
        sngCustomers = Range("Customers").Value
        
        If sngCustomers > 20 Or sngCustomers < 0 Then
            MsgBox strMessage, vbOKOnly, "Error"
            Range("Customers").Select
            Exit Sub
        End If
    
    End If

I placed it in a private sub under the worksheet .

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

I'm sorry, I don't quite understand what you mean by that.

It didn't work on my end. I am honestly very new with VBA so my entire spreadsheet needs a lot of work. That is one component of the spreadsheet and it is located at the top of the sheet. The purpose of the sheet is for the end user to input number of customers and painting ID and the worksheet will automatically generate the remaining inventory. If the inventory is below a minimum level, the user will be notified through a message. The entire spreadsheet is not working but I figured I would take the errors step by step in order to complete the project. The painting ID's are treated similarly to number of customers as the ID's range between 100-110. The code is posted below:

Option Explicit

Dim sngCustomers As Single
Dim sngPainting As Single
Dim sngCanvases As Single
Dim sngAprons As Single
Dim intWhite As Integer
Dim intYellow As Integer
Dim intOrange As Integer
Dim intRed As Integer
Dim intPurple As Integer
Dim intBlue As Integer
Dim intGreen As Integer
Dim intFlesh As Integer
Dim intSienna As Integer
Dim intBrown As Integer
Dim intBlack As Integer

Private Sub cmdReset_Click()
    Range("Activity").Select
    Selection.ClearContents
    Range("Customers").Select
End Sub


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim strCurrentCell As String
    Dim strB3 As String
    Dim strB4 As String
    Dim strMessage As String
    strCurrentCell = ActiveCell.Address
    strB3 = "$B$3"
    strB4 = "$B$4"
    
    'Data entry controls

    strMessage = "Check your entry for Customers," _
        & " there can only be up to twenty" _
        & " customers per class period."
        
    If strCurrentCell = strB3 Then
        'Assignment of Values from Work Sheet Variable
        sngCustomers = Range("Customers").Value
        
        If sngCustomers > 20 Or sngCustomers = "" Then
            MsgBox strMessage, vbOKOnly, "Error"
            Range("Customers").Value = ""
            Range("Customers").Select
            Exit Sub
        End If
    
    End If
    
    strMessage = "Check your entry for Painting," _
        & " the options range from 100-110."
    
    If strCurrentCell = strB4 Then
        'Assignment of Values from Work Sheet Variable
        sngPainting = Range("Painting").Value
        
        If sngPainting > 110 Or sngPainting < 100 Then
            MsgBox strMessage, vbOKOnly, "Error"
            Range("Painting").Select
            Exit Sub
        End If
        
    End If
   
    If strCurrentCell = strB4 Then
        'Assignment of values from worksheet to variables
        sngCustomers = Range("Customers").Value
        sngPainting = Range("Painting").Value
        sngCanvases = Range("Canvases").Value
        sngAprons = Range("Aprons").Value
        intWhite = Range("White").Value
        intYellow = Range("Yellow").Value
        intOrange = Range("Orange").Value
        intRed = Range("Red").Value
        intPurple = Range("Purple").Value
        intBlue = Range("Blue").Value
        intGreen = Range("Green").Value
        intFlesh = Range("Flesh").Value
        intSienna = Range("Sienna").Value
        intBrown = Range("Brown").Value
        intBlack = Range("Black").Value
        
        'Calculation of Inventory based on Customers
        sngCanvases = sngCanvases - sngCustomers
        sngAprons = sngAprons - sngCustomers
        
        'Calculation of Inventory based on Painting
        If sngPainting = 100 Then
            intWhite = intWhite - 0.5
            intPurple = intPurple - 0.5
            intBlue = intBlue - 0.5
            intGreen = intGreen - 0.5
            intBrown = intBrown - 0.5
            intBlack = intBlack - 0.5
        End If
            
        If sngPainting = 101 Then
             intYellow = intYellow - 0.5
             intRed = intRed - 0.5
             intPurple = intPurple - 0.5
             intBlue = intBlue - 0.5
             intBrown = intBrown - 0.5
        End If
             
        If sngPainting = 102 Then
            intWhite = intWhite - 0.5
            intYellow = intYellow - 0.5
            intOrange = intOrange - 0.5
            intRed = intRed - 0.5
            intPurple = intPurple - 0.5
            intFlesh = intFlesh - 0.5
            intSienna = intSienna - 0.5
        End If
            
        If sngPainting = 103 Then
            intRed = intRed - 0.5
            intPurple = intPurple - 0.5
            intSienna = intSienna - 0.5
            intBrown = intBrown - 0.5
        End If
            
        If sngPainting = 104 Then
            intWhite = intWhite - 0.5
            intYellow = intYellow - 0.5
            intOrange = intOrange - 0.5
            intBlue = intBlue - 0.5
            intFlesh = intFlesh - 0.5
            intBlack = intBlack - 0.5
        End If
            
        If sngPainting = 105 Then
            intWhite = intWhite - 0.5
            intBlue = intBlue - 0.5
            intGreen = intGreen - 0.5
            intSienna = intSienna - 0.5
            intBrown = intBrown - 0.5
            intBlack = intBlack - 0.5
        End If
            
        If sngPainting = 106 Then
            intWhite = intWhite - 0.5
            intRed = intRed - 0.5
            intBrown = intBrown - 0.5
        End If
            
        If sngPainting = 107 Then
            intWhite = intWhite - 0.5
            intYellow = intYellow - 0.5
            intOrange = intOrange - 0.5
            intRed = intRed - 0.5
            intBlue = intBlue - 0.5
            intGreen = intGreen - 0.5
            intSienna = intSienna - 0.5
        End If
            
        If sngPainting = 108 Then
            intWhite = intWhite - 0.5
            intOrange = intOrange - 0.5
            intRed = intRed - 0.5
            intBlue = intBlue - 0.5
            intGreen = intGreen - 0.5
            intFlesh = intFlesh - 0.5
        End If
        
        If sngPainting = 109 Then
            intWhite = intWhite - 0.5
            intYellow = intYellow - 0.5
            intRed = intRed - 0.5
            intPurple = intPurple - 0.5
            intBlue = intBlue - 0.5
        End If
            
        If sngPainting = 110 Then
            intWhite = intWhite - 0.5
            intYellow = intYellow - 0.5
            intBlue = intBlue - 0.5
            intBlack = intBlack - 0.5
        End If
        
        'Advice regarding minimum inventory points
        If sngCanvases < 400 Then
            MsgBox "Reorder Canvases"
        End If
        
        If sngAprons < 400 Then
            MsgBox "Reorder Aprons"
        End If
        
        If intWhite < 128 Then
            MsgBox "Reorder White Paint"
        End If
        
        If intYellow < 128 Then
            MsgBox "Reorder Yellow Paint"
        End If
        
        If intOrange < 128 Then
            MsgBox "Reorder Orange Paint"
        End If
        
        If intRed < 128 Then
            MsgBox "Reorder Red Paint"
        End If
        
        If intPurple < 128 Then
            MsgBox "Reorder Purple Paint"
        End If
        
        If intBlue < 128 Then
            MsgBox "Reorder Blue Paint"
        End If
        
        If intGreen < 128 Then
            MsgBox "Reorder Green Paint"
        End If
        
        If intFlesh < 128 Then
            MsgBox "Reorder Flesh Paint"
        End If
        
        If intSienna < 128 Then
            MsgBox "Reorder Sienna Paint"
        End If
        
        If intBrown < 128 Then
            MsgBox "Reorder Brown Paint"
        End If
        
        If intBlack < 128 Then
            MsgBox "Reorder Black Paint"
        End If
    End If

End Sub

All of the ranges are defined in the workbook. Range B3 = Customers within my working spreadsheet page.

I copied your code into Excel and it is still not working correctly. The message displays just the same way as mine had originally, but it does not force the user to remain on the original cell.