@Aam_jmp,
Could you possibly share the code ? Typically, the error occurs when The parameter you are passing to the function is not a list. But since you are not getting the same error in JMP 13, I am assuming there is something else at play here. Also, could you do the following:
Assuming List1 is the parameter you are passing to the function, add a Is List() to test if it is a list indeed.
Is List(List1);
If the result is 0, then try N Items(List(List1)) to see if that will fix the issue in JMP 12
Best
Uday