arpwatch: 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/arpwatch - from package: arpwatch

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

On Debian possible default full paths are:

/usr/sbin/arpwatch - from package: arpwatch

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

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