cssshrink.sh: 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/share/roundcubemail/bin/cssshrink.sh - from package: roundcubemail

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

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