By default, SteamOS is read only. Which means pacman won’t work.
I (sadly) don’t have a steam deck, so I can’t confirm this will work. But try following this. This will set up pacman on the steam deck so you can use it and then you can try installing fluidsynth again with pacman sudo pacman -S fluidsynth.
According to Valve, whatever you install may get wiped by the next update. So keep that in mind. And may get your Steam Deck into a bad state or compromise your data if you are not careful.