Allow an app from outside the Play Store on Android TV
Android TV asks before it installs anything that did not come through the Play Store. The permission goes to one app at a time: you are allowing that one app to install something, not opening the whole TV up.
Try the first. If it works you are done and can ignore the rest. Go on to the next one only if the previous one did not work.
Most TVs need nothing beyond the first.
1From the message on screen
When the TV says the app is not allowed to install unknown apps, choose Settings on that message.
A list of apps opens. Turn on the one you are installing from, such as Simple Downloader or a file manager.
Press back and the install continues.
Only if the first way did not work
2From Settings
With no message to click, the same list can be opened by hand. TVs put it in different places, so try these in turn:
Settings → Apps → Security & Restrictions → Unknown sources Settings → Device Preferences → Security & Restrictions → Unknown sources Settings → Apps → Special app access → Install unknown appsA TV whose Settings has a search box gets there fastest: search for unknown.
Only if neither of the above worked
3Switch on developer options
For when the list is missing, your app is not in it, or its switch will not move.
Some devices hide the permission until developer options is on: the Google TV Streamer, the Chromecast with Google TV and a few TV brands. Older Android TV boxes rarely need this.
In Settings, find About. It sits under System on Google TV, under Device Preferences on older Android TV, and in the main list on some sets. It is the same screen whichever way you reach it.
Scroll to the bottom. The last line is Android TV OS build (just Build on older devices). Select it and press OK on the remote repeatedly.
The TV starts counting down, along the lines of "You are now 4 steps away from being a developer". Keep pressing until it says you are a developer; seven presses in all.
"No need, you are already a developer" means it was on already.
Now go back to the first or second way; the permission is there now.
Is this safe?
Yes. This is what you are allowing
Installing apps from outside the Play Store is a standard Android feature; every other app store relies on it.
The permission applies to the one app you switch on, so nothing else gains the ability to install. Google Play Protect keeps checking the device as before.
Only switch it on for apps you trust, and feel free to switch it off again once Webcric is installed.