Hi Ruksana,
If account based COPA is active and if the cost element gets an account assignment to a PA segment during direct FI postings then system needs to post to both account based and costing based COPA. You cannot just post only to account based COPA if both types of COPA are active. Either you post to both or you do not post to COPA at all in which case you need another account assignment object like cost center, IO etc.
If there is an account assignment to PA segment then that cost element needs to be part of the PA transfer structure FI so that the costing based COPA posting can happen.
OKB9 is only required for the PA segment account assignment. If you do not want PA segment to be assigned you need to find another cost object.
regards
Waman