send_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:

/usr/sbin/send_arp - from package: piranha

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

On Debian possible default full paths are:

/usr/sbin/send_arp - from package: fake

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

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