arpd: 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 CentOS possible default full paths are:

/usr/sbin/arpd - from package: iproute

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/arpd - from package: iproute2

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

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