arpfetch: 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/arpfetch - from package: arpwatch

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

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