Thank you for reaching out for assistance through the Community. The Invalid link message is common if you have logged out of the registration form while accessing the link. So I recommend you to clear the browser, re-login into your account and access the link to continue filling the form.
If the problem still persists, I recommend contacting the school to confirm the link you are accessing is the correct link. If it is not, the school should be able to provide the correct link. If you need anything else let me know and I will be happy to help!