Hello Saurabh,
This behaviour occurs when your user (who is attempting to add the authorization for another user) itself does not have "Admin" authorization for the item. According to the program design only administrators can assign authorizations to other users.
Depending on the exact scenario (e.g. if you use "Add" or "Add Multiple", what kind of authorization you are assigning etc) you may get either error message "The specified authorization change is forbidden" or error message "The user XYZ is not authorized to change the specified authorizations" when trying to do so.
Best regards,
Francesco