Create alert in Grafana dashboard using Cloudwatch datasource

Hello, I am trying to create an alert for CloudWatch metrics in Grafana, I create query A and B with different metrics, and in query C expression,I referenced query A and B, and when I create alert I’ve got an error:
"tsdb.HandleRequest() response error &{ValidationError: Error in expression 'queryA': ID 'a' not found
Because Grafana doesn’t support referencing another query
How can I put 2 difference metrics in 1 query? Or How can I create an alert?

Can you confirm: are you using the new Unified Alerting platform or the legacy platform?