Use a function to test for the property.
If false you can then create the property in the else argument.
Code:
If chkPrp(tdf.Name, fld.Name) = True Then
Code:
Function chkPrp(tbl As String, fld As String) As Boolean
On Error GoTo chkPrp_Error
Dim chk As Variant
chk = CurrentDb.TableDefs(tbl).Fields(fld).Properties("Description")
chkPrp = True
On Error GoTo 0
Exit Function
chkPrp_Error:
If Err.Number = 3270 Then chkPrp = False: Exit Function
MsgBox "Error " & Err.Number & " (" & Err.Description & ") in procedure chkPrp, line " & Erl & "."
End Function