Hi!
You probably has two cloud accounts, and the workspace you are trying to register is already associated to this other account.
You will need to enter on that account and remove it, otherwise, you will not be able register with this new account.
Each Rocket.Chat installation has a Unique ID, and once it get’s tied to a cloud Account, it cannot be associated to a different account.
Let me know if this helps.
Also, there is a “way around”, which involves cleaning all cloud info from your Rocket.Chat workspace.
But this NOT is a suggested NOR ideal solution, as you would need to clear all Cloud_* settings values directly from Mongo DB settings collection.