lpunlock: 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 CentOS possible default full paths are:

/usr/bin/lpunlock - from package: expect

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/lpunlock - from package: expect

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

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