Are you using a cracked account? This error seems to be a common problem when logging in with cracked accounts.
Manacube is a premium server, and only supports premium accounts.
To join the server you can either direct connect and enter this ip: play.manacube.net or put it into add server. If you are wondering how to join kitpvp, once you log ontk the server you need to right click to compass and find the kitpvp logo/button.
It's recommended to use the IP pvp.manacube.net when playing on kit. This will make glitching in slabs and stairs a less frequent occurrence. (happens when you hit a slab/stair at a weird angle)
Manacube is on Java and bedrock, however, it's through minehut on bedrock, and as far as I'm aware, you can only get custom servers on mobile, and PC bedrock editions. It is recommended you join through the PC's "Java Edition" for the best experience.
To join the server you can either direct connect and enter this ip: play.manacube.net or put it into add server. If you are wondering how to join kitpvp, once you log ontk the server you need to right click to compass and find the kitpvp logo/button.