Hi Rachel,
Remove the first '\down to avoid the message error.
Here's a possible code:
\version "2.19.81"
\language "english"
pedaldn = \markup \concat \vcenter \center-align { D \tiny\natural }
pedalan = \markup \concat \vcenter \center-align { A \tiny\natural }
pedaldf = \markup \concat \vcenter \center-align { D \tiny\flat }
pedalgf = \markup \concat \vcenter \center-align { G \tiny\flat }
up = \change Staff = "treble"
down = \change Staff = "bass"
Treble = { s2.*2 }
Bass = \relative c {
\clef bass
\time 6/8
gf16 ef' gf \up bf ef ef \down gf,, ef' bf' \up c ef ef |
\down f,, c' a' \up c f a \down bf,, f' bf \up df f bf |
}
dyn = {
s2.\<-\pedalgf -\markup \concat \vcenter \center-align { C \tiny\natural }
s4.-\pedalan-\pedaldn s4-\pedaldf s16 s\!
}
\score {
\new GrandStaff
<<
\new Staff = "treble" { \Treble }
\new Staff = "bass" { \Bass }
\new Dynamics \with { \revert TextScript.font-shape } { \dyn }
>>
}
Cheers,
Pierre