Uncaught NetworkError: Failed to execute 'importScripts' on 'WorkerGlobalScope'
importScript() must be placed outside of a function. For your case you should use fetch(url). You should also add async to each function, and use it this way:
let message = fetchMyTournament(message).then(function(result){return result;});
For my case i was importing PouchDB like this:
importScripts("//cdn.jsdelivr.net/pouchdb/5.3.1/pouchdb.min.js");
The url should start with proper http/https.
So changing to this solved the problem:
importScripts("https://cdn.jsdelivr.net/pouchdb/5.3.1/pouchdb.min.js");