arp-scan: 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/arp-scan - from package: arp-scan

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

On Debian possible default full paths are:

/usr/bin/arp-scan - from package: arp-scan

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

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