“Unknown chord” for slash chords

By default ChordPro uses a built-in set of chord diagrams. These include all major chords, minor, seventh and most other common chords, buth these do not include slash chords. One of the reasons is that opinions on slash chords vary.

If you want to use slash chords, you need to define them first, either in your song file, e.g.:

{define: C/B frets x 2 2 0 1 0}

or in a config file.

See also define directive.