David Harris Well in that case, I'm as lost as you are.  If you're a subscriber and logged into YouTube with that account then seems you should be able to comment.  I tried. Good luck

David, curious, do you subscribe to the channel and are you signed in when this happens?

Had the same issue. Wes Severson was looking into it and mentioned that the platform provider was going to be adding features. You might try putting your video on something like YouTube, G drive, Dropbox, etc and then posting a link to it.

Are you watching on your phone or computer, not that it should make a difference. I thought you had to "join" the chat or "subscribe" to the channel.

These are good exercises