Data Validation List Error - vba

Question

I'm trying to add a data validation pulldown list, and I've been debugging it for a while to no avail. I'm getting a Run-time error 1004, Application-defined or object defined error. The error occurs in Formula1 setting part of the Validation.Add statement.
I've tried using a string reference to a named range, a string reference to a standard range, and, as shown below, a comma delimited list string generated from the list on the worksheet as shown in the code below. I have checked the list string with Debug.Print and got the expected result.
Sub addPT_Validation()
Dim sValidationList As String
Dim cell As Range
For Each cell In ThisWorkbook.Names("PT_Puldown").RefersToRange
sValidationList = sValidationList & cell.Value & ","
Next cell
sValidationList = Left(sValidationList, Len(sValidationList) - 1)
With ActiveSheet.Range("D14").Validation
.Add Type:=xlValidateList, _
AlertStyle:=xlValidAlertStop, _
Operator:=xlEqual, _
Formula1:=sValidationList
.IgnoreBlank = True
.InCellDropdown = True
.ShowError = True
End With
End Sub
Thanks in advance for any guidance.

Answer 1

All your code is working (certainly could be optimized), all you need to do is delete the validation if it already exists.
If you try to add validation to a cell which already contains one, you will get error 1004.
Sub addPT_Validation()
Dim sValidationList As String
Dim cell As Range
For Each cell In ThisWorkbook.Names("PT_Puldown").RefersToRange
sValidationList = sValidationList & cell.Value & ","
Next cell
sValidationList = Left(sValidationList, Len(sValidationList) - 1)
With ActiveSheet.Range("D1").Validation
'/Delete first., in case of any any existing validation
.Delete
.Add Type:=xlValidateList, _
AlertStyle:=xlValidAlertStop, _
Operator:=xlEqual, _
Formula1:=sValidationList
.IgnoreBlank = True
.InCellDropdown = True
.ShowError = True
End With
End Sub

Answer 2

I'd use the named range reference to build up the formula to put in Formula1 parameter of Validation object Add() method, like follows
Sub addPT_Validation()
Dim formula As String
With ThisWorkbook.Names("PT_Puldown").RefersToRange
formula = "'" & .Parent.Name & "'!" & .Address(External:=False)
End With
With ActiveSheet.Range("D14").Validation
.Add Type:=xlValidateList, _
AlertStyle:=xlValidAlertStop, _
Operator:=xlEqual, _
Formula1:=formula
.IgnoreBlank = True
.InCellDropdown = True
.ShowError = True
End With
End Sub

Answer 3

As pointed out by #cyboashu, your error is coming from the fact that you did not delete the validation before adding a new one.
With regard to your other issue of trying to pass the range to Formula1 directly:
Formula1 should refer to the range as a string.
eg:
Formula1:="=$G$2:$G$7"
or, using your variables:
Formula1:="=" & ThisWorkbook.Names("PT_Puldown").RefersToRange.Address
or, simpler:
Formula1:="=" & Range("PT_Puldown").Address