@Johnny_Ooooops
Did you enable “installation from unknown sources”?
If not, here’s a guide:
Go to the main Home page on your homescreen and tap the Settings icon.
Select the Security & Privacy category (older models might say Security), and turn on the Apps from Unknown Sources toggle.
If tapping Apps from Unknown Sources brings up a list of apps, don’t do anything. You may receive a popup when installing APKs, where you’ll tap the Allow button when prompted.