VB code doesn't work when sheet is protected

I have several cells that use vlookup to insert values based on a selection from a dropdown in a different cell. I locked the cells with the formulas and protect the sheet to preserve unintended formula corruption. However, when I protect the sheet, VB code that allows multiple values in a handful of unrelated cells stops working. I have very little VB knowledge/experience. Here is the code for multiple values:

Dim Oldvalue As String
Dim Newvalue As String

Application.EnableEvents = True
On Error GoTo Exitsub
If Target.Column = 13 Or Target.Column = 8 Or Target.Column = 14 Or Target.Column = 15 Or Target.Column = 18 Or Target.Column = 21 Then
If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
GoTo Exitsub
Else: If Target.Value = "" Then GoTo Exitsub Else
Application.EnableEvents = False
Newvalue = Target.Value
Application.Undo
Oldvalue = Target.Value
If Oldvalue = "" Then
Target.Value = Newvalue
Else
If InStr(1, Oldvalue, Newvalue) = 0 Then
Target.Value = Oldvalue & vbNewLine & Newvalue
Else:
Target.Value = Oldvalue
End If
End If
End If
End If
Application.EnableEvents = True
Exitsub:
Application.EnableEvents = True

Cells.Replace What:=Chr(10), Replacement:=";", LookAt:=xlPart, SearchOrder _
:=xlByRows, MatchCase:=False, SearchFormat:=False, ReplaceFormat:=False

End Sub

I used to have Activsheet.Unprotect in the beginning, and Activesheet.protect at the end, but I removed it because users had to keep unprotecting the sheet too many times and it was both frustrating and unproductive.

Thank you in advance.

Regards,
Nick

Try to protect this way, which allows macro's to run

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Originally Posted by JEC.

Try to protect this way, which allows macro's to run

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JEC, you rock! Thank you, thank you, thank you. This has been a thorn in my side and for the users as well. I appreciate you taking the time to help others.

Regards,
Nick

p.s. I tried to reply yesterday, but I don't see it in this thread so I am trying again.

Nice! You're welcome!
Cheers!