Shopping cart Error in transmission

Hi

i got an error messsage Shopping cart 10001xyz (PReq. 008xxxxxyz): XE020 Item category B not allowed with document type.

but shopping cart has been approved,after approved i had got this error.follow on documents not created due to this error.

so kindly suggest me where i have to check for this issue.awaiting for your specific inputs.

Regards

sss