Hi, I assume that you're working on Classic Scenario and using PI to create PO in ECC side.
You must activate FEH in order to get the errors otherwise you will need to debug the proxy to get the error messages.
After you activate FEH, you may use /n/SAPPO/PPO2 transaction to get the detailed messages from bapi.
Please if your scenario is different explain in detail so we can help you.
Best Regards!