What is the message?
you need to look carefully at the options it gives. The APK is not an officially signed APK (because Google does not allow lewd games to be on the appstore), so the default option when you try to install the game is to not install it. There should be something like “more option” and the install option should be inside
make sure you allow install third party apps https://www.youtube.com/watch?v=fk5R8KEFC-8