First, thank you very much for the library, and for looking at this.

According to:
https://www.musicxml.com/tutorial/the-midi-compatible-part/chords/
the first note in a chord should NOT have a tag.

I stumbled upon this when "flat.io" rejected the xml.

The following modifications seem to work:

music/music01 % diff -c creating.jl tst7.jl
*** creating.jl 2020-05-03 20:51:56.000000000 -0400
--- tst7.jl 2020-05-05 15:42:24.000000000 -0400

*** 62,68 ****

measure1_notes_guitar = [

G Major chord for a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 16, chord = Chord()),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 16, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 16, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 16, chord = Chord()),
--- 62,68 ----

measure1_notes_guitar = [

G Major chord for a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 16),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 16, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 16, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 16, chord = Chord()),

*** 72,78 ****

measure2_notes_guitar = [

G Major chord for half a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 8, chord = Chord()),
--- 72,78 ----

measure2_notes_guitar = [

G Major chord for half a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 8),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 8, chord = Chord()),

*** 80,86 ****
Note(pitch = Pitch(step = "G", alter = 0, octave = 4), duration = 8, chord = Chord()),

G Major chord for half a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 8, chord = Chord()),
--- 80,86 ----
Note(pitch = Pitch(step = "G", alter = 0, octave = 4), duration = 8, chord = Chord()),

G Major chord for half a bar

! Note(pitch = Pitch(step = "G", alter = 0, octave = 2), duration = 8),
Note(pitch = Pitch(step = "B", alter = 0, octave = 2), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "D", alter = 0, octave = 3), duration = 8, chord = Chord()),
Note(pitch = Pitch(step = "G", alter = 0, octave = 3), duration = 8, chord = Chord()),