How to enable Apps from Unknown Sources on an Amazon Fire TV or Fire TV Stick
Turning on Apps from Unknown Sources on an Amazon Fire TV or Fire TV Stick allows you to sideload apps onto the device from within another app, like a file browser. Enabling the Apps from Unknown Sources option is not required when sideloading apps via ADB or a PC utility. It is only necessary to enable this option if you want apps, which are already installed on your Fire TV or Fire TV Stick, to have the ability to install additional apps.
From the Fire TV or Fire TV Stick’s home screen, scroll down to “Settings”.
Next, scroll to the right and select “System”.
Next, scroll down and select “Developer Options”.
Then select “Apps from Unknown Sources” to turn the option to “ON”.
Lastly, select “OK” on the popup message which appears.