The service "fos_user.mailer" has a dependency on a non-existent service "templating"

In Symfony 3.4 and FosUserBundle 2.0, add a service mailer into the fos_user config:

fos_user:
    db_driver: orm # other valid values are 'mongodb' and 'couchdb'
    firewall_name: main
    user_class: AppBundle\Entity\User
    service:                               # this lines
        mailer: fos_user.mailer.twig_swift # this lines
    from_email:
        address: "%mailer_user%"
        sender_name: "%mailer_user%

I just experienced the exact same problem. The funny thing is that I created a Symfony 3.3 project just before creating a Symfony 3.4 project and the Symfony 3.3 project didn't have this problem. So they must have removed the templating component for the 3.4 release.

To solve your problem, you will have to install the templating component using composer:

composer require symfony/templating

Then, add the following configuration under the framework key in your config.yml:

templating:
    engines: ['twig']

Update: I recently had to start a new Symfony 3.4 project with FOSUserBundle and discovered that I only had to add the above configuration to my config.yml file (as was mentioned in a comment below).