I was able to join one with no issues. Do you have any blocking software running that might interfere with any of the scripts?
Unable to join live tournaments

Thanks for your response, Martin. As it turns out, the problem persisted for 4-5 hours and then just went away ... I have no idea why. Will just chalk this up to one of the mysteries of the computer universe. All's well now. :-)

I'm now having exactly the same issue. Doesn't seem to be any way of resolving it. I'm hoping that my problem goes away in a few hours also but I have contacted the Chess.com helpdesk to make them aware also. I'll post their response when received.

I can join tournaments but when I get the message to start and go to the tournament screen (white screen with chess board visible behind), I just get booted back to the tournament screen (black screen with list of tourneys I can join) and the tournament I was due to play in has disappeared. Any ideas?

The initial tourney tab just shows Upcoming tourneys. So, if you get bounced when or just before the tourney starts, then you would need to to change the option to In Progress. Though, once they are in progress, you can't join.

I can play some tourneys and other times I just get bounced. So don't think it is a settings issue.
Been OK all week and then today got bounced a few times.

This man plays chess at a 900 level but quickly, I recommend if you want to see random moves played quickly against 2500s, watch this mans games. He even wins afew on time, never position.

same problem ... either they fix this or I end my premium membership !!!
What happens when you try?
So how does one join a tournament?
https://support.chess.com/article/621-how-do-live-tournaments-work-where-can-i-join-one
I'm a diamond member and frequent live tournament player (just about every day!) All of a sudden starting today I am unable to join any live tournaments at all. Every time I try, I receive a message stating "Registration not available".
I've tried everything suggested in threads written by folks having this problem (close previous tournament windows, restart computer, clear cache, etc.) but nothing seems to work.
Is anyone else out there experiencing this, or have any suggestions for fixing it? Thanks in advance!