Ariadne GraphQL Subscriptions: "Could not connect to websocket endpoint ws://localhost:3001/"

Question

I was following this twilio blog on graphql subscriptions using ariadine. But when itry to make a subscription in the playground im getting the error:

{
  "error": "Could not connect to websocket endpoint ws://localhost:3001/. Please check if the endpoint url is correct."
}

Here is the subscription that I'm sending

subscription {
  post {
    error {
      message
      field
    }
    post {
      postId
      createdAt
      caption
    }
  }
}

here are my typedefs

schema {
  query: Query
  mutation: Mutation
  subscription: Subscription
}

input PostInput {
  caption: String!
}

type Post {
  postId: String!
  createdAt: String!
  caption: String!
}

type ErrorType {
  field: String!
  message: String!
}

type PostResponse {
  error: ErrorType
  post: Post
}
type Mutation {
  createPost(input: PostInput!): PostResponse!
}

type Subscription {
  post: PostResponse
}

Here is my resolvers.

1. mutations

@mutation.field("createPost")
async def create_post_resolver(obj, info, input):
    try:
        post = Post(postId=uuid4(), caption=input["caption"])
        db.session.add(post)
        db.session.commit()
        for queue in queues:
            queue.put(post)  
            
        return{
            "error": None,
            "post": post
        }
    except Exception as e:
        return{
            "error": {"message":str(e), "field": "unknown"},
            "post": None
        }

2. subscriptions

@subscription.source("post")
async def posts_source(obj, info):
    queue = asyncio.Queue(maxsize=0)
    queues.append(queue)
    try:
        while True:
            post = await queue.get()
            queue.task_done()
            payload = {
                "post": post,
                "error": None
            }
            yield payload
    except asyncio.CancelledError:
        queues.remove(queue)
        raise

@subscription.field("post")
async def posts_resolver(payload, info):
    return payload

What maybe possibly the cause of this error.