You need to be more clear to select one of the following user-exits to have your checks.. You should be able to resolve your issue with one of these, most probably the first one.
CONFPP05 - PP order conf.: Customer specific enhancements when saving
CONFPP02 - PP order conf.: Customer specific input checks 1
CONFPP04 - PP order conf.: Customer specific input checks 2
Instead, if you want to have checks during GR then you need to explore user-exits in MIGO.
Regards
KJogeswaraRao