LISTTAG argument is not a FORALL alias variable
You used an invalid argument when you called the ListTag function. The ListTag function may only be passed the ForAll reference variable of the ForAll statement:
Dim Y List As String
ForAll X In Y
Print ListTag(ABC) ' Illegal
Print ListTag(X) ' Legal
End ForAll
Replace the invalid argument in the ListTag function call with the ForAll reference variable where ListTag appears.