Dear Shekhar,
1. Kindly check the field selection key ME21, ME21N and the key has been maintained with PO document type whether "Info record field" is maintained with required or optional.
2. If it is optional for above selection key then kindly check the message settings for application 06 with following message number 136,304,305,306,310,341 and 342 or find your error message and make it warning.
Regards,
Karna J