Hello Mark Quinn,
The screenshot shows that Halo believes your Microsoft account is still active on another device. Since a new account also fails, this may be an account-session issue on Halo’s side rather than a normal linking problem.
Please sign out of Halo on any other Xbox, PC, or handheld where the account was used, then fully close the game and restart the PS5. If the same message remains, submit a ticket through Halo's support site. and attach this screenshot.
Microsoft Q&A is a public community forum, so contributors cannot clear or inspect Halo’s server-side account sessions.