@aizat So, the first one is a bug in mattermost UI. You can see that when you do some actions in the channels UI alone, there are some errors in the browser console. More importantly, I am able to do those actions like unarchive via the CLI just fine.
As for the CLI, this mmctl is a binary that you now have to run on your PC/Mac. You can download it here - https://github.com/mattermost/mmctl/releases . I have put a small note on how to use it here - https://docs.cloudron.io/apps/mattermost/#command-line-tool