Thanks Kumar for understanding the Scenario and responding.
However my issue is :
a) As system is accessing same GBB/VAX (W/o Account Assignment) why is it asking for cost element when I activated account based COPA and giving error for COPA derivation. It should not go to COPA at all.
b) And to avoid that if I remove the account assignment check in T.code OMWN, what can be the possible impact.
Note : After unchecking the checkbox I was able to post the entry correctly.
Thanks,
Ankur Goel