How to get around block of unknown apps when "unknown sources" is enabled? I just bought a Samsung Galaxy Tab A and I want YouTube Vanced on the tablet. But it won't let me install it because it's an "unknown" app. 
Yes, I have enabled "unknown sources" and it still won't let me install the app. 

A side note it's not blocking only YouTube vanced, it's blocking all unknown apps.

What can I do to get around this?

Here's a screenshot of the message:
I turned off Play Protect &amp; went to Settings &gt; Security &gt; More Security Options and cleared the credentials and was able to successfully install YouTube vanced and other apps!