You signed in with GitHub, but this site is restricted to members of a specific organization. Your account is not in that organization, so you cannot access the content.
When does this happen?
When auth is enabled (e.g. NEXT_PUBLIC_AUTH_ENABLED=true), only GitHub users in the configured org can view the site. If you're not in that org, you'll see this page after signing in.
If you believe this is an error, contact the repository owner.