Question about fos_auth_server.yaml

[softrare]

Hi! I have a web application referring a user to a separate identity provider application to log in through username/password which sends back an access token and the user gets logged in. The identity provider application has a "dummy" 2FA implementation which I would like to replace, e.g. with an extra Google Authenticator step using https://github.com/scheb/two-factor-bundle.

In my fos_auth_server.yaml, I have the usual lines reading

fos_oauth_server:
db_driver: orm # Drivers available: orm, mongodb, or propel
client_class: App\Entity\Oauth\Client
access_token_class: App\Entity\Oauth\AccessToken
refresh_token_class: App\Entity\Oauth\RefreshToken
auth_code_class: App\Entity\Oauth\AuthCode
service:
user_provider: fos_user.user_provider.username
options:
access_token_lifetime: 10800
supported_scopes: user

beneath that I have this

authorize:
    form:
        type:               App\Form\AuthorizationType
        handler:            App\Form\Handler\TwoFactorAuthorizationFormHandler
        name:               fos_oauth_server_authorize_form
        validation_groups:
            # Defaults:
            - Authorize
            - Default

the author of scheb/two-factor-bundle says I don't need this because on successful OAuth authentication I should be directly transferred to his 2FA form but if I remove this I get this error: https://ibb.co/8cFy3yS

Can anyone understand what causes this error? I am starting with Symfony and this is really something I have trouble with right now. Thanks a lot in advance for any efforts!