This has been resolved now.
This is more an authorisation issue.
Goto SU01
Enter the user id
Click on Personalisation TAB
Look for the below
Specifies the authorization level of the user in the approval BBP_WFL_SECURITY
Double click.
The security level should be maintained.
I have maintained for the perticular user and now the user can edit the PO while awaiting for approval.
But if you want to enable this functionality for all the users the Authorisation consultants might be able to do this setting centrally some where so every user can edit the PO while awaitinig for approval.
Thanks for the guys who has tried to help me.