It's a known issue some itch games have with the latest macOS versions.
The fastest solution is to download the game through the itch App
Alternatively, follow those steps:
- Unzip the downloaded file.
- Press Command + Space Bar on your Mac Keyboard. Type in “Terminal” When you see Terminal in the Spotlight search list, click it to open the app.
- copy paste the following line in the terminal window, paying attention to copy the last white space as well
xattr -d com.apple.quarantine
- drag and drop the StarBoy.app in the terminal window and hit enter.