When I try to run it, it throws an error saying .dll not found. As you haven't provided the .dll file, nobody will be able to play this unfortunately. I've made the same mistake myself in the past, feel for you.
Ahh man. This is my first game jam, so I wasn't too sure what to do. I used Unity. Should I have zipped the entire folder and submitted that instead?
Yes, the whole folder that is created by the "build" is necessary to run the game. Just make a ZIP file out of the folder and upload it.
Someone else here had the same problem and reuploaded his game (which was then linked as "late submission"), but I see a chance for you to fix this.
I tried to reupload it through the dashboard but there is a message saying I can't upload anything new until the jams over. Oh well.
