Hello Bhuvan,
Did you recently apply any SAP OSS notes ? or upgraded your system ? For me this looks to be a case of inconsistency in the system due to missing code.
Function module ME_APO_INITIALIZE_PO has the importing parameter IT_ATPFIELD_AB that had been added as part of one of the Service Pack releases. So, for sure this is brought by a note and it is clearly missing.
Please raise a ticket to SAP
Thanks & Best Regards,
Babu Kilari