Hi Deepak.
You can verify is the cost center was posted on document running tcode SE16N on the FALGFLEXA table, put the document number to do research.
After you can trying to post the new document through Tcode FB50L choosing or not a legder specific, then verify the document was posted correctly running the Tcodes SKb1 or Se16N with FAGLFLEXA table again.
Kind Regards
Jandoval Nascimento