"PKDownloadError error 8" upon Catalina update
I have been pulling my hair out with the same problem over the past few days, and have isolated the PKDownloadError error 8 issue - I was able to solve this via 2 methods:


use my iPhone's hotspot as the wifi for the update (note: this is
slower AND may be very expensive depending on your cellphone plan)
change the DNS settings on my home router to OpenDNS
(https://use.opendns.com/), allowing me to use my home internet
(faster &amp; cheaper)