winpopup-send: 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/winpopup-send - from package: kdenetwork

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

On Debian possible default full paths are:

/usr/bin/winpopup-send - from package: kopete

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

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