lilymidi: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/lilymidi - from package: lilypond

You may install the required package with command apt-get install {package}

Error in other languages:
lilymidi: bevel nie gevind nie
lilymidi: comandă negăsită
lilymidi: comando não encontrado
lilymidi: commande introuvable
lilymidi: command not found
lilymidi: käsku ei ole
lilymidi: không tìm thấy lệnh
lilymidi: komanda nerasta
lilymidi: Kommando nicht gefunden.
lilymidi: kommandot finns inte
lilymidi: komut yok
lilymidi: nie znaleziono polecenia
lilymidi: níor aimsíodh an t-ordú
lilymidi: no se encontró la orden
lilymidi: no s'ha trobat l'ordre
lilymidi: opdracht niet gevonden
lilymidi: parancs nem található
lilymidi: perintah tidak ditemukan
lilymidi: príkaz nenájdený
lilymidi: příkaz nenalezen
lilymidi: команда не найдена
lilymidi: командата не е открита
lilymidi: 命令找不到
lilymidi: コマンドが見つかりません