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/cssmin - from package: ycssmin
/usr/lib/node_modules/ycssmin/bin/cssmin - from package: ycssmin

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

On Debian possible default full paths are:

/usr/bin/cssmin - from package: cssmin

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

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