arpalert: 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/arpalert - from package: arpalert

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

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