ncrack: 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/ncrack - from package: ncrack

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

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