whatsup: 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/whatsup - from package: whatsup

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

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