I don't see all your config files, but you should check this:

1. In config/routes.yaml you have to remove api_login_check. This route is not yours, you don't have to implement it
2. Check if api_login_check exists in config/routes/security.yaml. Probably it is, but if no - just copy this part from your config/routes.yaml
3. Be sure your login firewall is the first one in the list. It's really important. Otherwise Symfony will try to find a custom controller to match api_login_check route.
4. As @Justinas mentioned in his comment, check if LexikJWTAuthenticationBundle is actually enabled in config/bundles.php. There should be a line like

Lexik\Bundle\JWTAuthenticationBundle\LexikJWTAuthenticationBundle::class => ['all' => true],