subs: 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/subs - from package: libsubtitles-perl

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

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