whatsdown: 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/whatsdown - from package: whatsup

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

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