Repeat Message box routine with IF condition and condnl formatting

Remove your Current Sheet change event code and no need of G26 cell since I covered it in the below code

To Add Excel VBA Code to a Workbook / This Workbook Module

1. Copy the code that you want to use
2. Select the workbook in which you want to store the code
3. Hold the Alt key, and press the F11 key, to open the Visual Basic Editor
4. In the Project Explorer, find your workbook, and open the list of Microsoft Excel Objects
5. Right-click on the ThisWorkbook object, and choose View Code
6. Where the cursor is flashing, choose Edit | Paste

Private Sub Workbook_SheetCalculate(ByVal Sh As Object)

If UCase(Sh.Name) = "SHEET1" Then
    With Range("C26")
        If .Value = "Delayed" Then
            MsgBox " Input Reasons for Delay. If On Time, Please enter 'NA'."
            .Offset(1).Select
            ActiveCell.ClearContents
            With Selection.Validation
                .Delete
                .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
                xlBetween, Formula1:="=Reason"
                .InCellDropdown = True
                .ShowInput = True
                .ShowError = True
            End With
        Else
            With .Offset(1)
                .Value = "NA"
                .Validation.Delete
            End With
        End If
    End With
End If
    
End Sub

Go through the attached file and place the cursor in H10 cell and press F2 and enter and check

Glad it helps you and thanks for the feedback and rep

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