我会采取一个猜测，但你安装了SonataDoctrineORMAdminBundle所描述的here？
要安装它，只需运行
composer require sonata-project/doctrine-orm-admin-bundle

我有类似的问题。当我尝试生成管理类时，我得到错误：

In AdminMaker.php line 295:

没有注册的模型管理器。
但是，我已经在bundles.php中安装并激活了捆绑包SonataDoctrineORMAdminBundle

Sonata\DatagridBundle\SonataDatagridBundle::class => ['all' => true],
Sonata\CoreBundle\SonataCoreBundle::class => ['all' => true],
Sonata\BlockBundle\SonataBlockBundle::class => ['all' => true],
Knp\Bundle\MenuBundle\KnpMenuBundle::class => ['all' => true],
Sonata\AdminBundle\SonataAdminBundle::class => ['all' => true],

出了什么问题？这个“模型管理器”是什么？Doctrine bundle也被安装和配置了...

doctrine:
dbal:
    # configure these for your database server
    # use postgresql for PostgreSQL
    # use sqlite for SQLite
    driver: 'mysql'
    server_version: '5.7'

    # only needed for MySQL
    charset: utf8mb4
    default_table_options:
        charset: utf8mb4
        collate: utf8mb4_unicode_ci

    url: '%env(resolve:DATABASE_URL)%'
orm:
    auto_generate_proxy_classes: true
    naming_strategy: doctrine.orm.naming_strategy.underscore
    auto_mapping: true
    mappings:
        App:
            is_bundle: false
            type: annotation
            dir: '%kernel.project_dir%/src/Entity'
            prefix: 'App\Entity'
            alias: App