This error can occur when the employee for whom you are creating the Back Pay was previously terminated and then reinstated. And the termination date falls within the Back Pay date range.
You can use Manual Calculation to enter the Back Pay or to process the Back Pay:
Go to Payroll, Maintenance, Employees, Employees.
Edit the Employee.
Select Termination Details.
Enter the Termination Date and Termination Reason.
Click OK to save.
Then go to Payroll, Transactions, Processing, Back Pay.
Enter the Back Pay details for the employee and click Process.
Once you have completed the pay, edit the employee record and remove the termination details.
It is recommended to create a new Employee record when an employee returns.