Vendor not permitted

Hello Experts,

There is a message ( Message class BBP_PD and message number 303 u2013 u201CVendor is not permitted). In program it is used as u201Cwarning messageu201D and system will give warning message when we are creating SC, but while processing PO it gives error message.

Becasue we can configure "warning" mesage to error message for "SC" but I want to know which program or FM or BADI is called when SC is being created and PO is being created.

Kindly help.

Thanking you

Regards

Sharan