Repeat Message box routine with IF condition and condnl formatting

Hi all,

I have a sample sheet which explains everything.

Two issues:
I want message box routine to repeat until valid value from range 'Reason' or 'NA' is entered.
Basic code:

Private Sub Worksheet_Change(ByVal Target As Range)
If ThisWorkbook.Sheets(1).Range("G26").Value = "1" Then
MsgBox " Input Reasons for Delay. If On Time, Please enter 'NA'."
'need help here to add - repeat messagebox routine until valid text from Range "Reason" and "NA" is entered
End If
End Sub

Second issue
If value in G26 is equal to 1, then cell C27 must get drop down menu options from Range 'Reason' and if value is zero then it simply indicates NA

Any help please
Kind regards