I have been having some problems with trying to download a free game on the Itch.io app - when I try to download it, the message in the title appears and I can't install the game. Sorry if this is a really common question or an easy fix that I'm just not seeing, and thanks in advance!

You did not ask a question. You described an issue.

For the Itch app this hoster is not compatible, so you cannot use the Itch app to install the game.

Use the website.

They might want to add that obvious solution to the error message maybe.

And be careful. I saw over 500 hacked accounts this year. But most of those games were hosted on Itch. Many devs use external hosters for various reasons. Mostly for size restrictions. But if you install a game yourself, you cannot use the Sandbox feature of the Itch app, which will protect you against many types of hacks.