This error means that the details you have for the parent or child - CRN or DOB are incorrect.
You will need to double check all the details with the parent and the parent may also need to contact Centrelink and double check that all their details are correct in the Centrelink systems.
Once you have the correct details you can re-enter them in the Children’s or Contacts Tab on the Accounts screen.
You will need to go to the CCMS Enrol tab, highlight and Edit the child's CCMS Enrolment and Click on Update now to send this correctly through to CCMS.