Unfortunately Unet is obsolete by now so I’m not sure how much you’ll gain by learning it now. They are coming out with a new Unity Networking system this year I believe.
As far as the issue you’re having, you have to make sure that when one player collects a cookie that the method associated with that is called for everyone. When I was testing it, I played as the client first to see if it worked, then tested it as the host. It’s a bit of a tedious process. Playing as the client lets you know if it’s working in a multiplayer environment while playing as the host gives you options to debug with the console.
