App not installed error on Huawei Mate 9 I downloaded apk files from Chrome. After the downloads completed, I pressed Install. But after 2~3 seconds, it just said App not installed. For example, I downloaded an app called "TutuApp". It won't install and doesn't give any information on why so I can fix it. 

Anyone knows how to fix this? 

My phone is Huawei Mate 9 with Android version 8.0.0.
I'm new to Android and a complete noob.
There's 'App not Installed' Error on Android on Stack Overflow.

For my case, Google Protect was on, so I was unable to install app.