uncpk: 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/uncpk - from package: xa

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

On Debian possible default full paths are:

/usr/bin/uncpk - from package: xa65

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

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