mscore: 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/mscore - from package: musescore

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

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