Cannot autowire service: Argument references class but no such service exists

My issue was a wrong namespace. File real position was App\Infrastructure\MySQL\Rubric\Specification But namespace was set to App\Infrastructure\Rubric\Specification

Result "[blah blah] but no such service exists".

Do you really need to add service definitions to services.yaml for third party vendor classes?

No, don't do that. My personal suggestion is: don't extend EntityRepository. Ever. You don't want your repository's interface to have method like createQuery or flush. At least, you don't want that if you consider a repository just like a collection of objects. If you extend EntityRepository you will have a leaky abstraction.

Instead you can inject the EntityManager inside your repository, and that's it:

use App\Entity\Activation;
use App\Repository\ActivationRepository;
use Doctrine\ORM\EntityManagerInterface;

final class DoctrineActivationRepository implements ActivationRepository
{
    private $entityManager;
    private $repository;

    public function __construct(EntityManagerInterface $entityManager)
    {
        $this->entityManager = $entityManager;
        $this->repository = $this->entityManager->getRepository(Activation::class);
    }

    public function store(Activation $activation): void
    {
        $this->entityManager->persist($activation);
        $this->entityManager->flush();
    }

    public function get($id): ?Activation
    {
        return $this->repository->find($id);
    }

    // other methods, that you defined in your repository's interface.
}

No other steps are required.

Starting from the 1.8 version of DoctrineBundle, you can extend your class using Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository instead of Doctrine\ORM\EntityRepository. The result will be the same, but this does support the autowire.

Example:

use App\Entity\Activation;
use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
use Doctrine\Common\Persistence\ManagerRegistry;

class ActivationRepository extends ServiceEntityRepository
{
    public function __construct(ManagerRegistry $registry)
    {
        parent::__construct($registry, Activation::class);
    }

    // ...
}