Describe the error
To import a elasticstack_elasticsearch_security_user resource, the documentation states:
terraform import elasticstack_elasticsearch_security_user.user <cluster_uuid>/**elastic**
The last elastic bit IS a variable, not a fixed word. As the documentation states, it is a bit confusing because most people think you need to use it instead of your resource name. Other resources don't have this fixed word, ie. elasticstack_elasticsearch_security_role.