Cortex 3.1.0 can't create ES index

Work Environment

Problem Description

I started Cortex with a very standard config based on the provided one in which essentially I just changed the url and index. But when i tried starting it up I got:

[error] o.e.d.DBConfiguration [|] ElasticSearch request failure: POST:/cortex_4/_search?scroll=60000ms
StringEntity({"version":"true","query":{"bool":{"must":[{"term":{"relations":{"value":"worker"}}},{"match_all":{}}]}},"from":0,"sort":[{"_id":{"order":"desc"}}]},Some(application/json))
 => ElasticError(index_not_found_exception,no such index [cortex_4],Some(_na_),Some(cortex_4),None,List(ElasticError(index_not_found_exception,no such index [cortex_4],Some(_na_),Some(cortex_4),None,null,None)),None)
[warn] o.e.d.SearchWithScroll [|] Search error
org.elastic4play.IndexNotFoundException$: null
	at org.elastic4play.IndexNotFoundException$.<clinit>(Errors.scala)
	at org.elastic4play.database.DBConfiguration.$anonfun$execute$2(DBConfiguration.scala:145)
	at scala.concurrent.Future.$anonfun$flatMap$1(Future.scala:307)
	at scala.concurrent.impl.Promise.$anonfun$transformWith$1(Promise.scala:41)
	at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
	at akka.dispatch.BatchingExecutor$AbstractBatch.processBatch(BatchingExecutor.scala:55)
	at akka.dispatch.BatchingExecutor$BlockableBatch.$anonfun$run$1(BatchingExecutor.scala:91)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at scala.concurrent.BlockContext$.withBlockContext(BlockContext.scala:85)
	at akka.dispatch.BatchingExecutor$BlockableBatch.run(BatchingExecutor.scala:91)

Which went away after I created the cortex_4 index manually. But why was it not created automatically? Does that not happen?
But the installation guide in no way mentions that an index needs to be created.

Possible Solutions

As I said I created the index manually, but:

1. If it needs to be created manually then it should be documented.
2. If it's not supposed to be created manually why does it complain that it doesn't exist?

Reproduce the startup failure using the provided application.sample configuration, Ubuntu 20.04, Cortex 3.0.1-1, and Elasticsearch 7.8.0. Compare the behavior with the Elasticsearch installation section of the installation guide and determine whether cortex_4 should be created automatically or documented as a manual prerequisite. Done means the expected index-creation behavior is established and the issue's requested fix or documentation is verified.