psad: 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/psad - from package: psad

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

On Debian possible default full paths are:

/usr/sbin/psad - from package: psad

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

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