Dear Paulo, The automatic fret generation code only works for instruments with monotonic (steadily increasing or decreasing) string pitches. Since the ukulele has non-monotonic string pitches, the algorithms do not work. See Known issues and warnings under https://lilypond.org/doc/v2.22/Documentation/notation/common-notation-for-fretted-strings#automatic-fret-diagrams
You can work around this by using the predefined fret diagrams for ukulele. \include "predefined-ukulele-fretboards.ly" See https://lilypond.org/doc/v2.22/Documentation/notation/common-notation-for-fretted-strings#predefined-fret-diagrams Hope this helps, Carl
