Multiline comment is being highlighted as Ruby code.

[vinibrsl]

In the following example:

class Score < ApplicationRecord
  has_one_attached :file

  enum key:  [:c, :db, :d, :eb, :e, :f, :gb, :g, :ab, :a, :bb, :b]

  =begin
    SCORE TYPES:
    (0) full: includes every instrument or voice in the composition
    (1) piano: sheet music for piano including both melody and accompaniment
    (2) vocal: version of the piano score with lyrics
    (3) tablature: specially for guitar and electric bass
    (4) lead: includes the melody, but the accompaniment is represented only in chords
    (5) chord: includes only the chords and lyrics for the song, aka cifra
    (6) percussion: sheet music specially for percussion instruments such as drums
  =end
  enum type: [:full, :piano, :vocal, :tablature, :lead, :chord, :percussion]

  belongs_to :song
end

The =begin and =end comment block isn't seen as a comment, but as Ruby code.

[sorbits]

You need to remove the leading whitespace before the =begin and =end lines:

class Score < ApplicationRecord
  has_one_attached :file

  enum key:  [:c, :db, :d, :eb, :e, :f, :gb, :g, :ab, :a, :bb, :b]

=begin
    SCORE TYPES:
    (0) full: includes every instrument or voice in the composition
    (1) piano: sheet music for piano including both melody and accompaniment
    (2) vocal: version of the piano score with lyrics
    (3) tablature: specially for guitar and electric bass
    (4) lead: includes the melody, but the accompaniment is represented only in chords
    (5) chord: includes only the chords and lyrics for the song, aka cifra
    (6) percussion: sheet music specially for percussion instruments such as drums
=end
  enum type: [:full, :piano, :vocal, :tablature, :lead, :chord, :percussion]

  belongs_to :song
end