On newer versions of macOS, they tightened up the permissions for the binary files that actually form the executable. I didn't need to do this to play the game, but I downloaded the compressed version via Mega to try it before posting this.
However, this will probably do the trick. Sorry, it means you need to launch the terminal app one time to fix this:
- Launch the Terminal.app from the Utilities folder that is inside your Applications folder.
- Type the following with a space included at the very end (that's important):
chmod -R 755
- Right click on the SunshineLove app and select "Show Package Contents".
- Click into Contents folder. You should see a MacOS folder as well as some other stuff.
- Now drag and drop the MacOS folder on top of the terminal window. This will paste in the path to the MacOS folder so you don't have to type it in manually.
- Press Return.
- Nothing will happen if this is successful, you'll just end up on a new terminal line.
- Now quit the terminal app and launch SunshineLove.
If that still doesn't work, then it is likely the version of the app you have on your computer is corrupted or perhaps you have some antivirus program stopping unknown applications from launching. I'd try downloading the app from another of the official locations or download the uncompressed version instead.