xnlock: 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/xnlock - from package: heimdal-clients-x

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

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