Hello! I hope you are well!
The error that you obtain usually appears because the master nodes in your Elasticsearch cluster have not passed the bootstrapping. In this stage, all the master nodes share their configuration to determine which are master eligibles and which are the initial master node.
This usually fails when the nodes can’t reach each other or because of a configuration error.
Please check that all the master nodes are running correctly and that the certificates are correct. Also, verify that all the nodes that have the tag node.master: true are included in cluster.initial_master_nodes: and their IPs in discovery.seed_hosts:.
could you share the /etc/elasticsearch/elasticsearch.ymland ~/instances.yml configuration file?