This problem is very common when the app developer didn’t have the itch client in mind. You can still download it via the ITCH website. (Take the url from the bar in the itch client and access the itch page of this game in your browser. You might need to log in to download a paid game.)

If any developer has complaints about this issue here is some context, (please let me know whether it works for you).

The itch client is a standalone app that provides a lot of convinience for the user.

• It provides (the possibility of) sandboxing apps, so they can’t steal passwords from my browser (currently opt in for the developer but that will change in the future).
• It automatically makes installable windows apps available under Linux (and maybe mac?) via wine without extra effort for the user or developer
• It provides a central place remove all the programms downloaded (you never forget some gigabyte big game in your download folder)
• It provides a way for users to kill badly behaving programms without opening a task manager
• Itch can collect crash logs that can help user and developer identify problems such as missing libraries (called *.dll on windows) which either a technically minded user can fix on their own or give detailed technical feedback to the developer
• Itch can also automatically install dependencies (such as programming language run-times) for the user when they are needed
• It does automatic updates of applications to the latest version in the background without user intervention
• Only updates the changes of apps when the developer pushed it using butler tool

To do this the itch client needs some information. I never published on Itch but i suspect those two articles have the information you need. You are probably just missing an App manifests if that doesn’t cut it look into the Game developer quick start and profit from the benefits of using butler: Compatibility policy#Platinum tier