This is similar to my experience. You'll find my original question and the "solutions" to the problem. It now appears that there is no sure way to fix what Microsoft has done, and all the responses from Microsoft on this thread are just "Here, try this" deflections. Microsoft will not offer refunds, and it won't reel back and fix whatever the underlying problem is so all of the purchasers can uninstall the failed version and installed the fixed version. As I am now convinced from my original post, I paid $250 for vaporware. So did all of you, here.
Another thing to note is that during this time of struggling to download/install and reinstall it... this was during the weekend. I later got an email from Microsoft claiming that "Order Placed" - Your Microsoft order (number) has been processed. Any downloads you brought, except preorders, are available now.
I'm guessing it might have taken a day or two for the store license key to be verified and processed by the Microsoft Account online? It might have just been ignoring it before that. If just purchased, wait till processed. I also have a desktop and laptop; it wasn't processed till signing onto both of them. I'm installing on the laptop, but it might have needed to confirm the desktop account. It said the license key is already taken by the laptop, but that's when the processed email arrived.
However, yeah, it's quite annoying and an odd glitch. The office installer itself should check for previous versions to at least warn you to uninstall first, and that the license is currently being processed, etc.