itch.io

Hmm, well a quick search seems to indicate that this has to do with how Macs handle application security. This is something you should be able to fix on your end by temporarily enabling all apps to run:

System Preferences -> Security & Privacy -> Allow apps downloaded from "Anywhere".

 Another suggestion I found online is to use the terminal window:

1. Open a terminal window (CMD + Space -> terminal);
2. Using the cd command, navigate to the place where the application is stored: cd <path_to_application>
3. Run chmod +x <application_file> to make it executable. If it doesn't let you, sudo it: sudo chmod +x <application>.

Hopefully one of these works for you.