It doesn't see the variable, major, because it is within curly braces, making it a literal. Try this slight rearrangement instead:
| minor major |
minor := #( 0 2 3 5 7 8 10 12).
major := #(0 2 4 5 7 9 11 12).
!Root +
(!Mode of: (Array with: (!Step of: major) with: (!Step of: minor) ))