rkill: 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/rkill - from package: pslist

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

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