To join an Enterprise workspace, your account can not be a part of any existing workspaces. If you would like to leave your current free workspace, you can do so on the Manage Members settings page of your account.
Steps to Leave Your Workspace
Press the three dot dropdown next to your email address, and then click Leave team. You will be restored to a single user account and from here you can accept your invitation to join the Enterprise workspace or log in using SSO.
If you are part of a paid workspace, please reach out to the owner of your workspace and ask them to remove your account. You can find the owner of your workspace in the Manage Members page.