Must implement interface > Drupal\Core\Entity\EntityTypeManagerInterface, none given in __construct() function
A bit late but the code from EventSubscriber has three arguments so your services.yml file should read:

services:
  custom_module.order_complete:
    class: Drupal\custom_module\EventSubscriber\PurchaseBuildRegistration
    arguments: ['@entity_type.manager', '@rng.registrant.factory', '@current_user']
tags:
  - { name: event_subscriber }