Hi Ali, this error is usually cause by incorrect work entries. You need to make sure that you do not have any conflicting work entries (check the conflicting filter on work entries) and also that you do not have any time off for any employees that are not validated. If you have a time off created but not approved or denied, you will not be able to create a payslip for that time period. 

It is also important to check if you have a valid contract for the employee. 

If you are trying to generate batch payslips, you can try debugging by creating separate payslips.