arp: 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:

/sbin/arp - from package: net-tools

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

On Debian possible default full paths are:

/usr/sbin/arp - from package: net-tools

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

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