whatis: 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/whatis - from package: man

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

On Debian possible default full paths are:

/usr/bin/whatis - from package: man-db

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

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