Videos not showing option to change thumbnail

I’m using the free tier of the audio/video encoding service. The videos I’m uploading will upload perfectly fine, but when I go to edit the videos, I can’t set thumbnails for them. I have attempted to regenerate the thumbnails for the video, and this hasn’t done anything for my problem.

So, how do I set the videos to generate thumbnails that users can assign to videos?

I still haven’t resolved this, and would appreciate assistance as soon as possible. It seems like a pretty common issue.