Allow an app from outside the Play Store on Android TV
Android TV checks with you before installing anything that did not come through the Play Store. The permission is given to one app at a time: you are letting that single app install something, not opening up the whole TV.
Start with the first. If it works, you are finished and the other two do not matter. Move on to the next way only when the one before did not work.
Most TVs need only the first.
1From the message on the screen
When the TV says the app is not allowed to install unknown apps, pick Settings on that message.
A list of apps appears. Switch on the one you are installing from, for example Simple Downloader or a file manager.
Press back and the install carries on.
Only if the first way did not work
2From Settings
Without a message to pick from, the same list can be opened by hand. TVs keep it in different places, so try these one after another:
Settings → Apps → Security & Restrictions → Unknown sources Settings → Device Preferences → Security & Restrictions → Unknown sources Settings → Apps → Special app access → Install unknown appsIf the TV's Settings has a search box, that is the quickest route: search for unknown.
Only if neither of the above worked
3Turn on developer options
For when the list is not there, your app is not in it, or its switch refuses to move.
A few devices hide the permission until developer options is on: the Google TV Streamer, the Chromecast with Google TV and some TV brands. Older Android TV boxes seldom need this.
In Settings, find About. On Google TV it is under System, on older Android TV under Device Preferences, and on some sets it is in the main list. It is the same screen whichever way you get there.
Scroll right to the bottom. The last line is Android TV OS build (just Build on older devices). Select it and press OK on the remote again and again.
The TV counts down, something like "You are now 4 steps away from being a developer". Keep pressing until it says you are a developer; seven presses altogether.
"No need, you are already a developer" means it was on all along.
Now go back to the first or second way: the permission is there now.
Is this safe?
Yes. Here is what you are allowing
Installing apps from outside the Play Store is a normal Android feature, and every other app store depends on it.
The permission covers only the one app you switch on; nothing else gains the ability to install. Google Play Protect goes on checking the device as it did before.
Switch it on only for apps you trust, and switch it off again once Touchcric is installed if you like.