Hi Radhesh
The backend system needs IW_BEP or SAP_GWFND.
It looks more like your system alias or trusted RFC definition is wrong.
Is your System Alias pointing to the backend system?
Is it set up to use the Current User?
Remember that what the inbox needs to do is to use the current user's id to read their worklist from the backend system - so that the user sees their own workitems. If you have it set to a technical user id that would be incorrect.
Check the steps in Masa's blog:
Fiori My Inbox - Step 3. Checking system configuration
This blog of Masa's is also helpful to check if the system alias definition itself is configured correctly. .
SAP Fiori LL18 - System Aliases definition for Fiori
Rgds,
Jocelyn