lightdm: 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/lightdm - from package: lightdm

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

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