I had the same problem but this may provide a solution. I recommend trying these two troublshooting actions after ensuring that you have the most up-to-date browser version. Brave just released an update, yesterday (04-15-2021, Version 1.23.71)
- https://www.publish0x.com/tomoyan/brave-rewards-no-brave-ad-notifications-fix-windows-10-xdjznv
- Here's a fix for those not receiving ad notifications. (WIN 10)
I am now receiving BAT but no pop-ups yet… good luck