Never seen this,but I'm guessing you need to pre-select the correct part body as the destination prior to performing the function. For example, maybe a geometric set is currently selected, but this is a part feature and so a part body must be currently selected before you start.
Do you have a 'Relations' set and Parameters set included in your PowerCopy? You might try adding a GeoSet with a Parameter Set and Releations set to the PC Definition.
@jasoncwells - nope, selection didn't solve the warning message
@dougsnell - Yes, I have the whole stuff, Parameters linked to Relations, design table, a reaction triggered at instantiation which is measuring an element and modify a parameter...
I tried to create a Geo Set and move all stuff inside as you said, it seems this is the solution for some PC, but not for this because I'm losing the design table and the reaction...anyway, I learned something new today, I have to rethink the PC, thank you Doug,