You are building formula for a target line item in REP04 module. However, your formula tells that your source line item 'Salary' is also in the same REP04 module because there is no module reference in front of Salary. If source and target line items are in the same module, you generally do not need to use a LOOKUP.
You might need to think whether your source line item is actually in another module. In that case, you can use either LOOKUP or SUM function to bring the number to REP04 module