I tried uploading my project again today and got a different error message this time. Interesting. However, I discovered something that might be the key to getting this to work.
When you go to the publisher console on yoyogames, select your asset and click "versions," you have the option to upload your asset straight from your web browser. I thought it might take the .yyz package but it didn't. However, I looked up the asset cache directory (from preferences > general settings > paths), and visited that directory. In there, I found a yymp file the size of my game package, so I tried uploading that to the publisher console. It took the file! However, when I tried to click "save" it gave an error saying "error 503 backend fetch failed" Apparently, this error means that the server isn't functioning. I suspect this is a reason why I got a different error when I tried uploading from GMS2. I'll try again later and hopefully I will be able to get through.