Appstore error 500 (com.apple.commerce.client)
this worked for me (what the user 'trs96' said):
https://www.tonymacx86.com/threads/appstore-the-operation-couldnt-be-completed-com-apple-commerce-client-error-500.270957/ 

basicly, just fire up a terminal session and run this command:

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