apt-key: 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-key - from package: apt

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

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