Grafana can't makes alert by expression with es datasource

We use grafana for alert configuration, which is based on conversion rate.
The data source of grafana is ES (Elasticsearch).

  1. First set up 2 queries in query,
    buyCnt: Number of page button clicks.
    pageCnt: is the number of page impressions
    Conversion via Expression: rate = $buyCnt / $pageCnt.

  2. In the alert, set the alarm according to the rate, and prompt an error: The datasource does not support alerting queries