arpsnmp: 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/arpsnmp - from package: arpwatch

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

On Debian possible default full paths are:

/usr/sbin/arpsnmp - from package: arpwatch

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

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