A common error is the year and term of matriculation entered. Matriculation marks your admission to the University; it should not be confused with graduation.
If matriculation term entered correctly and still experiencing issues it could be that the USN used is incorrect or that certain personal details (last name, date of birth) being used to register do not match what is held on your University central record.
If you continue to experience registration issues please submit a request with your details for further support.