Hi Pragathi,
From your post I assume following are the steps you being followed currently
1. Sales Quote Created, Saved
2. Now the SQ is being submitted for approval (can you confirm the transaction completed and were you able to see the approval notification to the respective approver?)
3. After this from the same SQ BO before save you are trying to Approve the SQ.
if the above process you are following it will not work.
Here make use of another intermediate BO which will be having the details of the SQ which are in status of "In Approval". On this BO's event before save try to call the Approve function of the SQ.
This intermediate BO should get populated with the SQ (In Approval) details through a PID asynchronous communication.
Regards
Hanu K