Elasticsearch service not running after downgrade from 7.16.2 to old version 6.4.2

Elastic Stack Elasticsearch
2

Welcome!

You can not downgrade.
If you have followed the recommandations, you probably have backups (snapshots) so you could:

  • Restore the cluster in 6.4.2
  • Upgrade to 6.8.23 which contains the security fixes you need

Otherwise, you need to fix the actual problem you have with 7.16. For this, you could start by sharing your logs.

Please read this about how to format your logs.