Quick note: When installing software — especially operating systems — open the laptop.
I almost always work on my Macbook Pro in clamshell mode. But it just doesn’t work when doing installs.
Some of the online hints are pretty scary. I matched problem 2 on this page but I wasn’t going to interrupt the installation without screaming …
So I was forced to think. And it came to me. Laptops aren’t meant to operate closed.