Magento 2 "Could Not Validate a Connection to Elasticsearch/OpenSearch": The Complete Fix
The Magento 2 "Could not validate a connection to Elasticsearch/OpenSearch" error has a short, fixable list of causes. This guide works through each one fix-first for Magento 2.4.4 to 2.4.9: the engine value, the Docker service hostname, port 9200, OOM on small boxes, TLS and auth, index prefixes, and the reindex that finishes the job.