light-locker-command: 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/light-locker-command - from package: light-locker

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

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