Can't install updates from macOS App Store
I have the same exact problem. I'm staying in a long term hotel and I had a couple of problems with their WiFi so I figured it had something to do with this. Yes, it was related but then something messed up around with my Apple ID. Then I could not even log in iTunes and AppStore and even my phone started acting weird.

The solution for logging again was typing this on the terminal:

defaults write com.apple.appstore.commerce Storefront -string \
    "$(defaults read com.apple.appstore.commerce Storefront | sed s/,8/,13/)"


(Source: Appstore error 500 (com.apple.commerce.client))

The problem is that my downloads and updates still never start and according to Apple Support everything is fine and it is just my network blocking downloads from AppStore! But I've tried 4G and a VPN interface and it is still the same!