This is wrong, it must be placed in the extensions directory, so the namespace must be
dgkim\calcmerit\notificationand the file should be in ext/dgkim/calcmerit/notification/some-name.php
Also the type definition should be in your extensions service.yml
You MUST NEVER modify any core files more in 3.1 nor put files anywhere apart from ext/
After adding the notification type to your extensions service.yml be sure to clear the cache.