I am also having a similar issue. I did not get an email with a serial number, my same serial number for 18/19/20 does not work even though the 2021 download came with a new activation key (which is usually the only thing that changes when a new update comes out), and i cannot find my serial number in my autodesk account. In fact, under products, it doesnt even show up as having a 2021 license, even though I was able to download and install 2021 through the revit page not through my products page. This is quite frustrating as once my 30-day trial is up, I can no longer open or edit any of my classmates' 2021 revit files since they are not backwards compatible with 2020. Someone from Autodesk really needs to get on this, because I have a few classmates who are having the same issue and have been given the runaround by 3 technicians over the phone support line.