You need to run lualatex with the --shell-escape option, e.g.: lualatex --shell-escape "Prova LetterBar".tex
If you're not running lualatex from a terminal (e.g. you use TeXworks or Scribus instead), you probably need to poke through some menus to figure out where it defines that it's running lualatex, and then type in --shell-escape there. (lualatex needs to run the gregorio program to convert your .gabc files, but because this is an outside program it wants specific permission to do so (--shell-escape) as a security precaution.)
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