If it's after you launch the game, I assume you also tried completely uninstalling and reinstalling? Since sometimes it just forgets to download certain files. That or just restarting your computer and the itch.io launcher
If the issue is you can't access the page or the banner to launch the game, and it says that error message at the bottom of the page, that's itch's problem, I usually go to my feed and then back or just reload the page. itch had some issues recently with pages not loading.