Enable Unknown Sources
By default, Android and iOS blocks the installation of apps that are not downloaded from the Android Apps section of Google Play and iOS sections of iTunes Store. This is an easy setting to change. If a device does not have access to Google Play Store or iTunes, you will need to allow 'Unknown Sources' installations. This is not uncommon and is required by several popular devices such as the Kobo Vox, Pandigital devices, and several Archos tablets.
*Note: After you have selected the APK you desire to download,
Click box to enable Unknown sources