@Miky In that case this is expected. The security plugin causes the errors. The plugin is initiated after the OpenSearch service is started. It needs around a minute or less to fully start and stabilize.
You can ignore these errors.
It would be a different story if these errors would continue infinitely. Then it could mean that your security config files are incorrect.
However, according to your previous outputs, you can successfully connect to the cluster using credentials. This means that the security plugin has been successfully initiated.
Regarding your APIs.
curl -X GET -u admin:admin “https://localhost:9200/.opendistro_security?pretty”
To see the content of the .opendistro_security index you must use admin certificates defined in the admin_dn of the opensearch.yml.
Alternatively, you can use the following API calls to check the content of the security configuration.
i.e.
Security config
curl --insecure -X GET -u admin:admin "https://localhost:9200/_plugins/_security/api/securityconfig?pretty"
Internal users
curl --insecure -X GET -u admin:admin "https://localhost:9200/_plugins/_security/api/internalusers?pretty"