you can fix the error by doing these steps
1. right click the "What-Lives-Below-Demo-Mac.app" file
2. click the "Show Package Contents" button
3. open the "Contents" folder then open the "MacOS" folder
4. right click the "What Lives Below Demo" and click the "Copy" button
5. open the terminal app by pressing COMMAND+SPACE and typing terminal.
6. in the terminal type "chmod +x" without the quotes and then press COMMAND+V after pressing space
7. press RETURN (enter). you do not need the terminal anymore so after the command completes you can just close the terminal.
8. double click "What-Lives-Below-Demo-Mac.app". there should be a popup saying that the developer cannot be verified press "Cancel"
9. open the "System Preferences" app and click on "Security and Privacy"
10. navigate to the "General" tab and press the "Open Anyway" button. there should be a popup that says:
"macOS cannot verify the developer of “What-Lives-Below-Demo-Mac.app”. Are you sure you want to open it?"
press the "Open" button and the game should launch. if the game does not launch then you can just double click the game file again.
Hope this helps --Floating Jacob