Heya, I'm on a Mac too and used to have a similar problem for a lot of games I find on itch.io. You can either:
1: Download the itch.io App (https://itch.io/app) and download the game through there instead of through the site, it should open properly
OR 2: Follow these steps
- Open the folder the game's in, right click on the game and select "Open Package Contents", then click Contents, click MacOS, you should see a file called "Yandere Heaven"
- Open Terminal, write the following command:
chmod u+x
(make sure there's a space after x)
then drag the "Yandere Heaven" file and drop it onto the Terminal window, then press Enter. The file's icon should change. Then try opening the game again.