cssdiff: 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/cssdiff - from package: crm114

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

On Debian possible default full paths are:

/usr/bin/cssdiff - from package: crm114

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

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