问AWS Cloudwatch Alert Terraform

EN

Error applying plan:

1 error(s) occurred:

* aws_cloudwatch_metric_alarm.alarm_name: 1 error(s) occurred:

* aws_cloudwatch_metric_alarm.alarm_name: Creating metric alarm failed: ValidationError: Exactly one element of the metrics list should return data.
    status code: 400, request id: xxxxxxxx-xxxxxxx-xxxxxx\n\nTerraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure."

resource "aws_cloudwatch_metric_alarm" "elb_monitor" {
  alarm_name                = "openam-elb-monitor"
  comparison_operator       = "GreaterThanOrEqualToThreshold"
  evaluation_periods        = "2"
  threshold                 = "1"
  alarm_description         = "UnHealthyHostCount for openam elbs"
  insufficient_data_actions = []
  metric_query {
      id = "elb_unhealthy_host_count"
      metric {
          metric_name = "UnHealthyHostCount"
          namespace   = "AWS/ELB"
          period      = "120"
          stat        = "Maximum"
          unit        = "Count"
          dimensions  = {
              LoadBalancerName = "development_lb"
          }
      }
    }
}