external-ip: 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/external-ip - from package: miniupnpc

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

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