wnnkill: 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/sbin/wnnkill - from package: freewnn-jserver

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

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