Hi Santosh,
It could be possible that the number ranges are not defined or altered. Please check the path in SPRO(in SRM) for the number range of backend follow-on-documents.
IMG : Supplier Relationship Management -> SRM server-> Cross Application Basic Settings->Number Ranges-> SRM server number ranges->Define Number ranges for shopping carts and follow on documents.
Please check if number ranges are maintained properly for PO. After maintaining the details, try to push the shopping cart again. Check in RZ20 for the error details further(if it occurs again).
Hope it helps.
Regards
Bash