unpigz: 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/unpigz - from package: pigz

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

On Debian possible default full paths are:

/usr/bin/unpigz - from package: pigz

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

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