Weird Apollo server error-could not connect to endpoint

I have defined my schema for query Messages, resolver and mutation, also the subscription. Followed the docs for Apollo server subscription but it keeps giving the same error.
I tried to use the Subscribe server from the docs but nothing works
please see here the code and further explanation of this apollo server subscriptions issue:

https://stackoverflow.com/questions/65642947/weird-apollo-server-error-could-not-connect-to-endpoint

Start with the Stack Overflow report linked in the issue and compare the described schema, resolver, mutation, and subscription setup with the Apollo Server subscription documentation. Reproduce the “could not connect to endpoint” error and determine what information or project change is needed for the subscription endpoint to connect.

Written by the indexing model from the issue text.