Hi Meghal,
I think you have not implemented a BADI.
Fiori My Inbox - Step 6. Implement the BADI for updating the decision
It handles actual action logic right after you selected a decision button.
Most of users in Line of Business prefer individual app like Approve PO, Approve PR, etc,
I would use My Inbox for other workflow scenarios which are not on the standard Fiori apps.
Regards,
Masa / SAP Technology RIG