There are two possible reasons for this error:
Register your cert through the official employer portal at myfuturefund.ie. Use your existing Revenue Online Services (ROS) certificate.
Your payroll doesn't require new AEPNs every period. In this case, there are no employees that need updating in this pay run.
If an employee isn't enrolled as expected, contact NAERSA. Enrolment is managed by NAERSA and communicated to Payroll through the AEPN.