That provider account is already linked elsewhere.
Why a third-party identity can't be on two Citadel accounts.
What happened
The third-party identity (GitHub, Google, etc.) you attempted to link is already attached to a different Citadel account. A provider identity binds to exactly one Citadel account at a time — re-binding would silently break sign-in for the other account.
What to try
- Check whether you already have a second Citadel account. Try signing in to it via the provider directly (Continue with X on the sign-in screen) — if you land on a different dashboard, that's the account holding the identity.
- If the other account was a stale duplicate, sign in there, disconnect the provider via Settings → Profile → Linked accounts, then retry the link from your primary account.
- If you no longer have access to the other account (lost email, etc.), contact support — we can verify ownership of the provider identity out-of-band and re-bind on request.