Help with Message Box

It seems you are doing more than just a message box? Why is there an addition to column F in the code?

Try this, I have no idea what you wish to do with Case Else part though.

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rng As Range
    Set rng = Target.Parent.Range("E5:F1000")
    If Target.Count > 1 Then Exit Sub
    If Intersect(Target, rng) Is Nothing Then Exit Sub
    Application.EnableEvents = False
    Select Case Target.Parent.Cells(Target.Row, "G").Value
        Case Is < 12
            MsgBox (Range("E1") + " has less than 12 hours rest"), vbQuestion + vbOKCancel, "Exceedance"
        
        Case Else
            'Target.Offset(, -1).Value = Target.Offset(, -1).Value + Target.Value
    End Select
    Application.EnableEvents = True
End Sub