xss-lock: 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/xss-lock - from package: xss-lock

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

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