You don't need to import SubscriptionServer from subscriptions-transport-ws. Here is a simple GraphQL server template for you.

server.ts

import "reflect-metadata";
import express from "express";
import { ApolloServer } from "apollo-server-express";
import { buildSchema } from "type-graphql";
import http from 'http';

const main = async () => {
  const app = express();
  const httpServer = http.createServer(app)

  const apolloServer = new ApolloServer({
    schema: await buildSchema({
      // Insert your resolvers here
      resolvers: [],
      validate: false
    }),
    // Not required but if you want to change your path do it here. The default path is graphql if "subscriptions" field is not passed
    subscriptions: {
      path: "/subscriptions",
      onConnect: () => console.log("Client connected for subscriptions"),
      onDisconnect: () => console.log("Client disconnected from subscriptions")
    }
  });

  apolloServer.applyMiddleware({ app, cors: false });
  apolloServer.installSubscriptionHandlers(httpServer)

  httpServer.listen(4000, () => console.log("Server ready at http://localhost:4000"))
};

main().catch((err) => console.error(err));

resolver

import {
  Arg,
  Mutation,
  Resolver,
  Root,

  // Required for subscription to work
  Subscription, 
  PubSub,
  PubSubEngine
} from "type-graphql";

@Resolver()
export class MessageResolver {
  @Subscription({ topics: 'NOTIFICATIONS' })
  newNotification(@Root() payload: string) : string {
    return payload
  }

  @Mutation(() => String)
  async sendMessage(
    @Arg('name') name: string,
    @PubSub() pubsub: PubSubEngine
  ): Promise<string> {
    pubsub.publish('NOTIFICATIONS', name)
    return name
  }
}

Note To answer your question, it's highly likely that Typescript isn't reading the file where you wrote your subscription resolver, resulting in no GraphQL schema being created. Be sure to add the following lines to your tsconfig.json file. Sometimes, vscode auto imports paths and add them to this line.

  "include": [
    "./src/**/*.tsx",
    "./src/**/*.ts"
  ]