For the app to install properly, you have to allow apps from unknown sources. To do so, open the SETTINGS of your device, scroll down and tap on SECURITY. Scroll down again to find UNKNOWN SOURCES and allow the installation of apps other than the Play Store.
Your phone or tablet will ask you to confirm. Tap on OK to do so.
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As soon as the download is complete, pull down your notification screen and tap on the download notification.
Tap on INSTALL.