cleancss: 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/cleancss - from package: cleancss

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

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