Thanks for reaching out and sharing your question with us.
To help us troubleshoot this effectively, could you please clarify exactly what is happening?
Is it an account issue or a connection issue?
Login: Are you unable to sign in to the Roon ARC app entirely?
Connection: Are you signed in, but unable to connect to your Core due to a network error?
Diagnostics: A screenshot of the error would be very helpful. Could you please go to your Roon Remote app on your computer/tablet, navigate to Settings > Roon ARC, run the connection test, and share a screenshot of the result here?