This problem may be outlined as, "You downloaded apk file and try to install it but install button does not pressing/ working and cancel button works." The main reason is some apps most likely screen adjuster apps blocking it. The most likely solution comes in the form of security applications interfering with the installation, package incompatibility with the device or screen filter applications used to lower the device's brightness below the minimal limit of the Brightness settings. So, best solution is, just uninstall that apps. Still having issues, try AppInstaller. Here is list of such apps, if you found more just comment here. Lux Screen Adjuster Halo Velis Auto brightness Soft buttons light Zulu's Droidmote server Easy Eye JuiceDefender Ultimate Night Mode Darker, etc.