If Focus is running, and a session has started, but blocking isn't working—check to ensure Automation Permissions are enabled.
When Focus launches it will automatically prompt you to enable Automation permissions.
If you don't click Allow, you can enable permissions manually by going to System Preferences > Privacy & Security > Automation and enabling all the browsers under Focus:
Focus Automation Permissions
Restart Focus, start a new Focus session, and your distractions should be blocked in all enabled browsers.