cssparse_py3: 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/cssparse_py3 - from package: python3-cssutils

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

Error in other languages:
cssparse_py3: bevel nie gevind nie
cssparse_py3: comandă negăsită
cssparse_py3: comando não encontrado
cssparse_py3: commande introuvable
cssparse_py3: command not found
cssparse_py3: käsku ei ole
cssparse_py3: không tìm thấy lệnh
cssparse_py3: komanda nerasta
cssparse_py3: Kommando nicht gefunden.
cssparse_py3: kommandot finns inte
cssparse_py3: komut yok
cssparse_py3: nie znaleziono polecenia
cssparse_py3: níor aimsíodh an t-ordú
cssparse_py3: no se encontró la orden
cssparse_py3: no s'ha trobat l'ordre
cssparse_py3: opdracht niet gevonden
cssparse_py3: parancs nem található
cssparse_py3: perintah tidak ditemukan
cssparse_py3: príkaz nenájdený
cssparse_py3: příkaz nenalezen
cssparse_py3: команда не найдена
cssparse_py3: командата не е открита
cssparse_py3: 命令找不到
cssparse_py3: コマンドが見つかりません