apache www-data

Try with following commands:

1. This will give you the name of apache user.
   ps -ef | grep apache | grep -v grep
   
   In most of the cases the user will be www-data.
2. Run these command after going into the /frontend/web folder: 
   sudo chgrp www-data ./assets 
   and 
   sudo chmod g+w ./assets/
3.