Solved - Can't install macOS Catalina; An Internet connection is required error

While trying to update my MacBook Pro macOS from High Sierra to Catalina, I kept getting "An Internet connection is required" error at the very end of the installation. Found a closed 2019 thread on this same problem very helpful: https://discussions.apple.com/thread/250739427

The key to solving this problem for me was to successfully install a security update for High Sierra first. This update had been failing to install. Following the suggestions in the referenced thread, I first tried to install the security update in Safe Mode (didn't work). Then I created a separate test account with admin privileges and used safe mode, then the High Sierra security update installed successfully. After the restart I used the test account again without Safe Mode and tried the Catalina installation again. No errors this time! I don't know if it would have installed successfully in my regular account, but regardless this sequence worked.