lightdm-kde-greeter: command not found

Have You tried accessing the binary with the full path?

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/lightdm-kde-greeter - from package: lightdm-kde-greeter

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