arpspoof: 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/arpspoof - from package: dsniff

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

On Debian possible default full paths are:

/usr/sbin/arpspoof - from package: dsniff

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

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