whois: 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/whois - from package: jwhois

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

On Debian possible default full paths are:

/usr/bin/whois - from package: whois

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

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