xasharma Flarum does not officially supports web login for search engines. I don't think anyone has ever attempted it, so it's a complete unknown as to whether it would work and what would the bot do with the javascript application once logged in.
The web login for human users happens via POST /login with the identification and password parameters. But it's protected by CSRF, so I'm not sure Google bot will be able to submit that form. Additionally it's a JSON response that will be returned and not a redirect, so this could also throw the bot off.
If that doesn't work, a custom extension could probably be done to implement a bot-friendly login page.
All that being said if all you want is the warning to get away, there are probably multiple solutions that don't involve giving Google any login information. I'm pretty sure you can blacklist pages from the Google search console itself.
