srt: 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/srt - from package: python-pysrt

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

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