Follow the below steps to enable maintenance mode in drupal 8 site

  1. Click on the Manage adminstrative menu
  2. Goto Configuration > Development > Maintenance Mode
  3. Check "Put site into maintenance mode" checkbox
  4. Add the Message to be displayed to user while putting site in maintenance mode
  5. Click on "Save Configuration" button

Access your site from other browser and verify if Maintenance mode message is displayed. If it's not displayed then clear your site cache and browser cache and then try reloading your site.