I don't think this function (unknown sources) comes into play when loading from google Playstore. Some devices will even show this in a message when side loading.
Weather it's true or not I never have checked on that, as I have never use the Playstore.
Some devices will even direct you to the correct menu on your device, to set that option, but only when attempting to side load.
Put it on a usb stick and use a OTG cable and then use a File Explorer to install it.
Also if you can not find unknown sources do this
install apps from unknown sources, you will need to Turn On "Apps From Unknown Sources" in the Developer options on your Fire tv Stick.
Navigate to the Settings section and Open My Fire TV. Select the About option. Tap the Fire TV Stick option repeatedly until you see the
message that you are a developer.
the fire stick is a 2nd gen. about 2 yrs old ( I think ), I have turned on apps from unknown sources etc, and installed some other apps that work ok. Currently have kokaleka v-2.1.2 which was installed when the stick was new, and it is working but I would like to update to the latest version. Not sure what OTG cable is but pretty sure I do not have one.
Are you trying to install from Google Playstore ?
Or are you trying to install from the apk file ? (I posted the link for the latest version of the apk/app in my last posted comment).
Is the error message seen when attempting to download the apk from that linked page ?
Or is the error message coming when you attempting to download from Playstore?
Or is the error message coming when attempting to install the apk onto the Firestick ?
1 you plug your fire stick power cord into 1
2 it has a usb port on it for a usb stick and you can put a apk on the usb and download of it (or you can use it for storage also.
3 you plug the last cord into your fire stick to give it power.
it's a very handy cable and they are cheap to buy.
finally have latest version loaded on firestick. It finally worked after loading update on firestick software. Loaded through downloader app on firestick using link from this site. (Was not able to download from playstore). All is ok now, thanks for the help