I had a similar thing occur with a previous iMac. The solution is to return to the Apple Store. When it happened to me and I returned the person said something like "Oops, Service forgot to finalize the installation." They were able to do it on the spot, using secret software, of course, and all was good.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.15.x,; iPhones, iPods and iPads galore!