Allow App Installations from Unknown Sources - Phone by Google
Your device and personal data are more vulnerable to threats such as Ransomware from apps downloaded from unknown sources. Here's some additional info about downloading from unknown sources and protecting your device against Ransomware.
From a Home screen, navigate to Settings .
Tap Apps & notifications.
Tap Special app access.
Tap Install unknown apps.
Select the unknown app then tap the Allow from this source switch to turn on or off .