The B2 documentation above is no longer accurate. The Duplicacy GUI is also outdated, making the B2 setup process very confusing.
Specifically: both the GUI and the documentation say that you need the account id and application key. So I created a new application key, then entered my account ID and application key… and it didn’t work.
It should really say that you need the application key id and the application key. (Or, alternately, you can use the account id and master application key, which could be less secure.)