python-cssmin: 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/python-cssmin - from package: python-cssmin

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

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