ifplugd: 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/sbin/ifplugd - from package: ifplugd

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

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