When Roon told you that your licence was already in use when you tried to log in, there should have been an “Unauthorise” button available.
Selecting this should remove the association between your Roon License and the old, defunct server and allow you to continue to complete the login process.
Thank you for reaching out to Roon Support — we’re glad to see you in our community!
Could you please let us know if you were able to unauthorize the old server by following the steps shared by @Wade_Oram? If you’re still encountering any issues, kindly attach a few screenshots of what you’re seeing on your screen so we can better assist you.