I cannot access the returning student registration form through powerschool. I am told that access was denied and I don't have authorization to view the page (error 403).
Have you contacted your school to see if your credentials are valid or if you account might need to be reactivated? Although not the same message you are seeing, you might find the following two Knowledge Base articles helpful:
If working with your school doesn't resolve the issue you are having, one of the articles does include instructions for opening a case with PowerSchool Mobile Support.
I hope that this is helpful.