Exporting from Unity is often more of a pain than it should be (this is one of the ways Godot has Unity beat haha). It’s good practice to build early and make sure everything works properly. As the commenter below stated, for Unity projects you need to upload a zip file with all the contents from the build folder it generates.

Just a heads up, the exe alone is not enough to play the game. You need to upload everything from your Unity build folder aside from that one file it explictly says not to upload