To turn Focused Inbox off (or on) in Outlook on the web, go to Gear icon , Options, Layout, Focused Inbox.
Click Settings at the bottom right. The Focused Inbox toggle is near the bottom of the Settings page. The change should take effect immediately.
Click 3 dots on the right, then Settings.
Toggle Focused Inbox off. You'll need to close and restart the Outlook app for the change to apply.