I'm an instructor using the free Canvas account. I have several students who have successfully enrolled in the class using the link I sent out but, shortly after, when they try to log in it now says "access denied" and they can't view the course. I've tried deleting and re-adding them to the course, having them use a different browser, use incognito mode, and clear their browser history/cache/cookies but the error message still pops up. How can I resolve this error?