Starting from version 1.2, the 2FAS browser extension uses system-native notifications.
Your browser may be configured to disallow notifications. Whether you use Windows or Mac, you can fix this problem with a few clicks.
On Windows, click the Start button and then Settings, select System, Notifications, and enable them at the top of the page. Make sure your browser notifications are also enabled.
On Mac, click the Apple icon in the menu bar and select System Preferences. Click Notifications & Focus. Find your browser (there will be two instances for each browser), and enable notifications for both.