Pradeesh,
I see two possibilities: either you've set some messages up to be hard errors, or you've put in an enhancement.
For the error messages, see http://scn.sap.com/thread/3319199, which explains the relevant error messages and how to configure them.
If it's an enhancement, you'll have to check with your ABAP team.
What's the message you're getting when you try to save the PO?
- Jeff