This worked for me! 1. go to settings and if your on android or any other devices, click files with a orange icon with a folder colored in white, click on it. 2. Scroll down on its app and you will see a bar saying "Install unknown apps" go to a little bit to the right and you will see a on and off button. Click the on button. And it will let you install gacha shine and other gacha mods! (Hope it helped!)