subtitlecomposer: 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/subtitlecomposer - from package: subtitlecomposer

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

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