<?php declare(strict_types = 1);

use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
final class User
{
	#[ORM\Id]
	#[ORM\Column]
	private int $id;
}

This error is reported by the phpstan-doctrine extension.

A Doctrine entity class is declared as final. Doctrine uses proxy objects for lazy loading of entity relationships. Proxies are generated as subclasses of the entity class, but a final class cannot be extended. This means Doctrine cannot create proxy objects for this entity, which can cause problems with lazy loading.

Remove the final keyword from the entity class:

 <?php declare(strict_types = 1);

 use Doctrine\ORM\Mapping as ORM;

 #[ORM\Entity]
-final class User
+class User
 {
 	#[ORM\Id]
 	#[ORM\Column]
 	private int $id;
 }

Starting with Doctrine ORM 3.4, lazy ghost objects are used by default for lazy loading. Lazy ghost objects do not require extending the entity class, so entities can be final. On older Doctrine ORM 3.x versions, lazy ghost objects can be enabled explicitly through the Configuration object:

$configuration->setLazyGhostObjectEnabled(true);