Why can't I install apk file.? Everytime I press Install,it keeps saying app not installed. I tried rebooting, restarting.. Still can't install.. But i can install apps on google play.. Just not on my files... I really need it.
Navigate to "System &amp; Device" -> "Additional Settings" ->  "Privacy" -> "Unknown Sources" (Enable this "Unknown Sources" option to allow installation of apps from unknown sources)



Adding more information as on date 11/14/2018 based on the comments: Apk related issue can occur due to the following. Please give it a try and let me know your observations.

Hoping that the APk file is not corrupted/infected and you are facing this issue with every APK file

1) Storage Location - Try checking if the apk is trying to install on Android device’s internal memory/storage or else where. By this we can ensure apk is getting installed in the correct path where you have access.

2) Check if SD Card is mounted or not in the Device
Try by going to Settings -> Storage -> Mount SD Card -> Restart device and try installing

3) Application Permissions - Software operations running in the background are set not to allow the installation of third-party software, it is also one reason why app not installed error on Android happens.

Go to Settings > Apps > Reset App Preferences/ Reset Application Permissions. After this, third-party software can be installed on your device.