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)