Use the entity type object

modules/recurring_events_registration/recurring_events_registration.module

@@ -40,7 +40,7 @@ function recurring_events_registration_entity_base_field_info_alter(&$fields, En
       ->setTranslatable(FALSE)
       ->setCardinality(1)
       ->setRequired(FALSE)
-      ->setTargetEntityTypeId('eventseries')
+      ->setTargetEntityTypeId($entity_type->id())
       ->setDisplayOptions('form', [
         'type' => 'event_registration',
         'weight' => 3,