Need help - Purchased a game in 2019 and it says product key was already used but it's not...

It does give me the option to install directly, but it still asks me for the code.. This is where I'm getting angry. I can give more screen shots, but I'm pretty certain the one I sent with me attempting to re-enter the 'required' code showed that already.

Yes, however the game isn't showing up in my account except for under the purchase history. I have checked both accounts I have and it is on neither of them. It shows under my purchase history, but not under my owned / redeemed games. This is very very
frustrating.

If it's showing in your purchase history, then it's on your account. You would need to sign into a console to see it in your library.

Here. This is all that happens without re-entering the code...

If it's not showing in your library, check the game in the store as well and see if it gives you the option to install it since you own it. If not, you'll want to contact support directly. http://support.xbox.com/contact-us