kwin: 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/kwin - from package: kdebase-workspace

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

On Debian possible default full paths are:

/usr/bin/kwin - from package: kde-window-manager

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

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