MaxDiff on the Fly

I am trying to work for the first time with the maxdiff on the fly, but my code is not working.

Background:
1) MaxDiff exercise name: Print
2) List of Maxdiff factors (13 members): PrintList
3) Want to recover the 8 most important factors in a constructed list.

My constructed list: Q2pListTOP ParentList: PrintList

Then to add most important factor, I am using the following code:
ADD (PrintList, MaxDiffRankAttLabel(Print,1))

I am getting an error for invalid arguments. Could you help me troubleshoting this?

Thanks!