I have downloaded a third party apk file for android phone but when try to install the app the install button not working but the cancel button working fine, I have tried lot of things like checking unknown sources and downloaded from different website but the result were same; apk app not installing, finally I got the solution its work perfectly with me hope this will also work with you, the solution is very simple just close all the background running apps and also clear all the recent app history and than try to install app, this will fix installation problem.