This is a very common problem that almost everyone faces while installing mods on their devices. However, there are certain things that you can do to fix this particular issue once and for all. First of all, go through the instructions provided in section 3 of this article carefully so that you can follow them properly. If that doesn't work at first, then make sure to restart your phone in safe mode (completely shut it down and turn if off) and then install Fallout Shelter Mod APK again.