dnsscan: 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/bin/dnsscan - from package: pdns-tools

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

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