whatweb: 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 Debian possible default full paths are:

/usr/bin/whatweb - from package: whatweb

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

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