Hi Krishna,
This could happen in the following case. You have a planning level with keys at aggregat level with attributes at lower level
eg PERPRODFAMILYCUSTGRP with keys PRODFAMILY and CUSTGRP. In this planning level , if you include attributes which are non unique for the root attribute, example PRODID, PRODSUBFAMILY which cannot be uniquely determined for root PRODFAMILY, then the data load will error with this message when you upload key figure at that planning level.
Hope this solves your issue.
Thanks,
raghav