Replica can not assigned shard in single node

Elastic Stack Elasticsearch
1

I wonder if the replica cannot assign a shard on a single node ( elsticsearch version 8.1.2 ).
I know that single node does not need replica but version 7.17.1 can assign shard in replica.

I've done the upgrade v 7.17.1 to v 8.1.2 and run reroute, but the problem still remains.

2

You cannot assign replicas on a single node cluster, Elasticsearch will never allow it as there's no redundancy offered by doing this.