In my case I have to add three periods (...) before on, something like:

query getOrderType($userId: String!) {
  getOrderType(userId: $userId) {
    ... on DeliveryType {
       delivery
       deliveryStreetNumberId
       pickupLocationId
    }
    storeId
  }
}

Doc: https://www.apollographql.com/docs/apollo-server/schema/unions-interfaces#querying-a-union