The PowerSchool Community is a great resource if you have any changes to be made while completing fields within the enrollment form. Each district, however, has a different registration process and can provide the best steps to move forward when enrolling students.
This issue usually occurs if the grade field on the form has not been updated correctly. I suggest you report this error to the district directly because they would be able to review and update the field and assist you to proceed with the registration.
You can check out the FAQs related to the enrollment process here.