# config/services.yaml

    sylius.fixture.example_factory.channel:
        class: App\Fixture\Factory\ChannelExampleFactory
        arguments:
            - "@sylius.factory.channel"
            - "@sylius.repository.locale"
            - "@sylius.repository.currency"
            - "@sylius.repository.zone"
            - "@sylius.repository.country"
        public: true
    
    sylius.fixture.channel:
        class: App\Fixture\ChannelFixture
        arguments:
            - "@sylius.manager.channel"
            - "@sylius.fixture.example_factory.channel"
        tags:
            - { name: sylius_fixtures.fixture }

# config/packages/sylius_fixtures.yaml
sylius_fixtures:
    suites:
        myt:
            listeners:
                orm_purger: ~
                logger: ~

            fixtures:
                locale:
                    options:
                        locales:
                            - 'de_DE'
                currency:
                    options:
                        currencies:
                            - 'EUR'

                geographical:
                    options:
                        countries:
                            - 'DE'

                channel:
                    options:
                        custom:
                            de_store:
                                name: "DE Store"
                                code: "DE"
                                locales: 
                                    - "de_DE"
                                currencies:
                                    - "EUR"
                                enabled: true
                                hostname: "localhost"
                                countries:
                                    - "DE"

//src/Fixture/Factory/ChannelExampleFactory.php
<?php


namespace App\Fixture\Factory;

use Sylius\Bundle\CoreBundle\Fixture\Factory\ChannelExampleFactory as BaseChannelExampleFactory;
use Sylius\Bundle\CoreBundle\Fixture\OptionsResolver\LazyOption;
use Sylius\Component\Channel\Factory\ChannelFactoryInterface;
use Sylius\Component\Core\Model\ChannelInterface;
use Sylius\Component\Resource\Repository\RepositoryInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class ChannelExampleFactory extends BaseChannelExampleFactory
{
    /** @var RepositoryInterface */
    public $countryRepository;

    public function __construct(
        ChannelFactoryInterface $channelFactory,
        RepositoryInterface $localeRepository,
        RepositoryInterface $currencyRepository,
        RepositoryInterface $zoneRepository,
        RepositoryInterface $countryRepository
    )
    {
        $this->countryRepository = $countryRepository;

        parent::__construct($channelFactory, $localeRepository, $currencyRepository, $zoneRepository);
    }

    public function create(array $options = []): ChannelInterface
    {
        $channel = parent::create($options);

        foreach ($options['countries'] as $country) {
            $channel->addCountry($country);
        }

        return $channel;
    }

    protected function configureOptions(OptionsResolver $resolver): void
    {
        parent::configureOptions($resolver);

        $resolver
            ->setDefault('countries', LazyOption::randomOne($this->countryRepository))
            ->setAllowedTypes('countries', 'array')
            ->setNormalizer('countries', LazyOption::findOneBy($this->countryRepository, 'code'));
    }
}

//src/Fixture/ChannelFixture.php
<?php


namespace App\Fixture;

use Sylius\Bundle\CoreBundle\Fixture\ChannelFixture as BaseChannelFixture;
use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;

final class ChannelFixture extends BaseChannelFixture
{
    protected function configureResourceNode(ArrayNodeDefinition $resourceNode): void
    {
        parent::configureResourceNode($resourceNode);

        $resourceNode
            ->children()
            ->arrayNode('countries')->scalarPrototype()->end()->end();
    }
}

php bin/console sylius:install -s myt

Argument 1 passed to Sylius\Component\Core\Model\Channel::addCountry() must implement interface Sylius\Component\Addressing\Model\CountryInterface, string given, called in /home/lucian/www/sylius/myt  
  /src/Fixture/Factory/ChannelExampleFactory.php on line 36