How to generate error message in fast formula for 'Coverage Upper Limit'

I want to add a custom error message incorporated into the 'Coverage Upper Limit' fast formula, if there is no benefit balance exist for the employee. How to add a pop message error message for an employee while trying to make enrollments for employee.

If balance is not available, now the upper coverage limit shows as '0'. But, instead of that I want to show a pop up message to inform the employee that there is no balance loaded for the employee.