I know posts regarding this issue already exist. But the solutions were always to report a problem and wait for Canvas team to solve the individual problem. That means the problem has persisted for years (the earliest post I saw was in 2015).
I reported the problem and the reply I got through email is "Canvas Support has seen such a large increase in volume that we are no longer able to provide support for all Free For Teacher help requests" and "For assistance with any other issue, please visit the Canvas Community".
I think it is irresponsible for Canvas to discard Free For Teacher instructors in this way. If you notify me in advance, I could choose not to use Canvas. But now I had student submissions that I cannot mark, and you tell me service is no longer available, it is unfair for my students and it certainly damages the reputation of Canvas.
I am sorry to hear that one of your students experienced an error when trying to upload a media file and that you cannot now grade it. I wish that our support staff were able to help you troubleshoot this problem. However, the reality is that during 2020 as use of both paid and free instances of Canvas saw a massive increase, we had no option but to update the Free for Teacher documentation, explaining what types of technical issues we could still help non-paying users with. To do otherwise while an order of magnitude more paying customers were needing help would have been irresponsible.
With kind regards,
Scott Dennis, Sr Dir Community.