arp2ethers: 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/arp2ethers - from package: arpwatch

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

On Debian possible default full paths are:

/usr/sbin/arp2ethers - from package: arpwatch

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

Error in other languages:
arp2ethers: bevel nie gevind nie
arp2ethers: comandă negăsită
arp2ethers: comando não encontrado
arp2ethers: commande introuvable
arp2ethers: command not found
arp2ethers: käsku ei ole
arp2ethers: không tìm thấy lệnh
arp2ethers: komanda nerasta
arp2ethers: Kommando nicht gefunden.
arp2ethers: kommandot finns inte
arp2ethers: komut yok
arp2ethers: nie znaleziono polecenia
arp2ethers: níor aimsíodh an t-ordú
arp2ethers: no se encontró la orden
arp2ethers: no s'ha trobat l'ordre
arp2ethers: opdracht niet gevonden
arp2ethers: parancs nem található
arp2ethers: perintah tidak ditemukan
arp2ethers: príkaz nenájdený
arp2ethers: příkaz nenalezen
arp2ethers: команда не найдена
arp2ethers: командата не е открита
arp2ethers: 命令找不到
arp2ethers: コマンドが見つかりません