In macOS 10.14 Mojave and newer, operating system updates have been moved from the Mac App Store to a dedicated panel withing System Preferences called System Updates.
Enable the OS to automatically update itself by performing the following steps:
- Go to Apple Menu -> System Preferences -> Software Update
- Click on the Advanced button at the bottom right of the Software Update window.
- Check all the boxes as you see in the image below, then click OK.