When I enter a multiplayer and I want to enter a room … it appears to me that there was an error, that I have to contact the host to have a kind of permission.
In order for us to provide better support, we need to see the log generated by Citra. This guide will walk you through how you can obtain the log file: How to Upload the Log File.
Did you verify your username/token? go to Emulation > Configure > General > Web and on username input your forum username and on token input your token. click on ‘Verify’ afterwards, if everything went ok you should be able to join rooms. you may have to re-open citra if its fails.