If App Crashes on Start up:
Enable Permissions for Storage & Phone
If App is in a White screen:
Clear Cache and force stop the app.
If App is detected as Unauthorized or modified:
Force Close the game!
Clear Cache, Force stop and if it's still not working unallow all permissions and reallow.
If app isn't downloading:
Make sure to install
SIGNED: If you don't have rooted device. If you don't know what root is you probably don't have root so install this one.
UNSIGNED: If you have root + LuckPatch Patchers installed.
If you don't know if you have root or not, most likely you don't have root.
(Thanks to @Anitakawaii for sharing his/her experience)
Go to the first message, the post. You should see some options that are locked. You have to like the post and the options to download should be unlocked. Download the signed one.