I use macOS and none of the games that I dowload work beacuse a screen apperes that doesnt let me open it saying that since it cant find the developer, how do I fix this? 

Try playing them through the app.

If you don't feel like downloading the app, attempt to run the app as normal, and after that screen shows up, go to system preferences, click on privacy and security, and it will say "(Game) was blocked because it cannot be identified" with a button to open it anyway. I hope this helps!

Also, once you've downloaded a macos app, you can control-click on it and in the resulting context menu select Open to run it (it'd be a nice itch.io feature to have these instructions displayed for macos apps downloads)

https://itch.io/t/1125819/unidentified-developer-problem-with-macos-and-itchio-g...

Apple has the tendency to block all applications not coming from the AppStore or being developed by verified developers.
This is a measure taken to make sure they keep earning money on you to make sure you are never target of malware.

What technicat says is as far as I know still true (I left Mac a few years ago, as Apple's paranoid attitude was getting on my nerves to be honest). You can by the way if you have a two button mouse also use the right mouse button.

By the way, Google also does this. I tried downloading and installing Android apps from itch.io on my phone via  Chrome and it gives the same type of warning telling you to stick to Google Play. But downloading with mobile Firefox works fine.

I used to use a Mac computer.

The old “unidentified developer” error. Open up Preferences/Settings.

Go to Security and Privacy. Closer to the bottom of the screen, it will tell you something like “[app name] was blocked from opening because it is not from an identified developer” and next to it will be a button that says “Open Anyways.” Click on it, and you’re all good. You may need to put in an administrator password, however.