SUMMARY The documentation says that for "Setting the keys in KMyMoney", "On the encryption page of the settings dialog, there is a drop down list where you can select the key that should be used for encryption and decryption." This should be corrected to point to the settings dialog of the XML plugin, which is where these keys are listed. It also doesn't explain what to do when the options there are greyed out – I still haven't figured out what am I doing wrong. Tried under Linux and macOS both.
This has caused confusion before already: https://forum.kde.org/viewtopic.php?f=69&t=151564
OK, looks like the keys are not getting enabled on macOS, despite the keys present in .gnupg folder. I can see them on Linux. Will probably file a separate bug report for that.
Are you sure that you have a pinentry program set up and working correctly. I have a vague memory of this causing problems with encryption. (I'll add this to my list for updating the handbook.)
*** Bug 425906 has been marked as a duplicate of this bug. ***