Once rewards are enabled, you’ll get ads in notification format, Sponsored image or Brave news. So make sure you enable Brave news.
For sponsored images on PC →
Click on the customise button on the new tab page → Backrground Image → Scroll to the bottom and enable Sponsored images.
On mobile →
Settings → New tab page section → toggle sponsored images to on position.
Enable Brave news in settings in Brave settings in ‘brave news’ section on both OSes