Acentra supports both cracked & premium accounts. This means that if you did not pay for Minecraft, you can still play on it.
The system we use allows us to support client cosmetics (so that you can see cosmetics on Lunar, Badlion Client etc) including other additional benefits. To do this, we have to ensure that Premium users cannot be taken by a Cracked user.
If you see this error when connecting through a Cracked Launcher, it means that the username you chose is a Premium Account.
You can check if an account is taken or not by using NameMC. Simply type in the Username to see if it's 'Available' OR 'Unavailable' To avoid getting this issue, simply pick a new username and try again. If you happen to continue to get this error and you're sure it is NOT taken, please open a ticket for support.