itch.io

Make sure you download the correct executable.  The one for MacOS. Other than that, I'm afraid I'm unable to help, as I don't have a Mac computer myself. Hopefully you get it solved!

I am using Mac on Big Sur and it worked first time

so if you cant open it using admin pass and such do this. Download the game. go to the .app file of the game. right click and view its properties. you should see a folder called content. copy that folder. after that create a folder called anything you want but with the .app extension ex. Game.app. go to the properties of that application you made and paste the contents folder in there. Run the app and enjoy.