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Situation: When attempting to install/upgrade a Android apps, you may receive a message saying: "Installing this app may harm your device. The app may be harmful to your device and personal information" or "Installation has been blocked. The app is harmful and can’t be installed" and can't install it.
Resolution: If you attempt to install an app that is not listed on the Play Store while app verification is turned on or Unknown Sources is checked, the device may block you install the app.
If you have Android v4.1 or above and want to install the unknown app, go to Settings>Security to disable Unknown source as shown in the screenshot.
If you have Android devices with old version, you may try this post: How to configure Verify apps settings in Android devices
