Symfony4 - 'app/config/config.yml' is missing?

Why not to create it config/packages/fos.yaml ? Or add in one of existing config/packages/foo.yaml ? https://symfony.com/doc/current/configuration.html

As Arnold Richmon indicated. Also, in the fos_user.yaml file you need to add config options as:

fos_user:
    db_driver: orm # other valid values are 'mongodb' and 'couchdb'
    firewall_name: main
    user_class: App\Entity\YourUserEntityClass
    from_email:
        address: "[email protected]"
        sender_name: "yoursendername"

for that problem you can try this:

Create fos_user.yaml file under config/packages folder then you can add the configuration code as usual, symfony4 will load it automatically.