Advanced Protection is a program offered by Google to those potentially in need of enhanced mobile-device security. The Mountain View giant has announced some new changes to its policies. They include a built-in blocker for side-loaded, non-Play Store apps.
https://www.notebookcheck.net/Google-s-Advanced-Protection-starts-to-prevent-the-side-loading-of-non-Play-Store-apps.458474.0.html