Well basically when I exported the project I selected windows, mac and linus so if it doesnt work then I dijt know why. Sorry for any inconveniences
When your in Unity you have to build the game one by one.
When you want to make game's for multiple platforms you have to click windows and build it then click mac then build it then click linux then build it and export all the builds to itch.io and choose mac for the mac build, choose windows for the windows build, and choose linux for the linux build and then when you post it people can play it on all platforms;)