apt-src: 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/apt-src - from package: apt-src

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

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