Some apps require special permission from your phone before they can be downloaded or installed. If such permissions are not set from your phone, it may cause the problem of apps not downloading. To resolve this, perhaps you should download your apps from the Google play store, or from any other verified app store whose apps are from verified manufacturers. This will arrest the need to alter the settings of your phone to disallow the downloading of apps from unknown sources.
Delete the account associated with your Google play store, restart your phone, and resync the account with your play store, if the downloading issue also occurs from apps that you seek to download from Google play store. 1e1e36bf2d