whois_psad: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/whois_psad - from package: psad

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