This error means that you’ve already created an alias matching the one you’re trying to add. Say that you created an email forwarder such as email@example.com. Because you’ve created this in the past, you’re no longer able to register that alias again.
You may want to check if a matching email exists, as this would be the issue. If it doesn’t exist and still throwing the error, please screenshot the error and I’ll notify developers.
I hope this helps you! Let me know if you need more info