IExpense error : The specified approver is not a user of the application.

Problem statement:  While creating the iexpense report user faces a issue that "The specified approver is not a user of the application. Please enter another approver. or select one from the list of values.

Diagnosis:
select * from per_all_people_f where full_name = &SupervisorName;
Take the person_id and apply as an input in the below query
select * from fnd_user where empolyee_id = &person_id;

The result of the query will be no data found

Soultion:
Create the oracle/FND user for the supervisor.
Navigation:  System administrator --> Define user

check the below query, if it gives result then the issue has been resolved. Now you can try to re-create the expense
report and you will be able to create the expense report this time

select * from fnd_user where empolyee_id = &person_id;