The error is more likely from how you packaged the game. Or how Construct 3 packaged it for you, depending. I was asking how you tried to upload exactly, and what error message you got.
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Im not sure if it was really an error because i didn't get any message. I uploaded it this way: Dashboard> Create new project> Upload file(s). I tried to download the game for myself and it was a c3p file. I opened it and Windows asked if i would like to use chrome to open the file, or Microsoft store.
Well, if you can't figure it out, ask in General Development. But the export command shouldn't be hard to find, it's pretty important.
However, this page from the Construct 3 manual says the export option is under Menu->Project. I just tried it online, and it's there.