Since SMMWE is a fan-made project and not available on the Google Play Store, you need to be careful about where you download the APK.
If you’ve ever dreamed of designing your own Mushroom Kingdom levels on the go, you’ve likely heard of . While Nintendo’s official version remains locked to consoles, the fan community has stepped up in a massive way. The release of the SMMWE 4.0.0 APK has become a "hot" topic for a reason—it’s the most stable, feature-packed, and visually stunning mobile fan-game ever created.
Go to your phone’s Settings > Security and toggle on "Install from Unknown Sources" to allow the APK installation.