Trigger error message in SRM Backend PO

Hi, experts.

We are in SRM702 version.  When user clicks on "Create a PO" button in RFx response, I should validate data and trigger an error message to SRM screen if it's necessary. So here is my question: is there a BADI to achieve this requirement? I have already tried BBP_DOC_CHECK_BADI and /SAPSRM/IF_BADI_SOA_MAPPING but it doesn't seems to work as I need.

Thanks in advance.