Here are some possibilities:
1) The user may have tried to login using an email address instead of his username. In this case he will need to click "Sign out and sign in with a different account", sign in with the username in the correct format, and complete any necessary password and MFA steps.
2) Saved activities in your web browser could cause this error. He can resolve this by logging in using a fresh browser or incognito window.
3) There may be an internal policy that is blocking that user. If that is the case you can create a new personal login ID to resolve the issue.
4) The user may be missing a license that the other users have. You can check for this and make sure the necessary licenses are assigned.
5) The user may have recently changed a password and is having a keychain-related issue .In this case you can try removing all Microsoft/Office items from the user's keychain or creating a new keychain.
6. If by chance he is using a student/educational subscription, he needs to sign in with his organization's email address and use his student account.
Let me know if any of these steps help.