Occurs in visual studio code, using pandoc citer extension. Typing '@' in the editor throws an error, reporting the file 'default.yml' is missing.
Resolution is to provide the file. First, find the pandoc data directory:
% pandoc -v
pandoc 3.1.12.3
Features: +server +lua
Scripting engine: Lua 5.4
User data directory: /Users/dave/.local/share/pandoc