How to use expressions in alerts

I would like to set monitoring alerts. Prometheus is already enabled on cluster lever and for each projects. Now I try to add more alerts with expressions.
I user following source to get the expressions:
For expample I take this one:

sum(container_memory_working_set_bytes{namespace="$namespace",pod_name=~"$podName", container_name!=""}) by (pod_name)

then i change it to

sum(container_memory_working_set_bytes{namespace="MYNAMESPACE",pod_name=~"MYPODNAME", container_name!=""}) by (pod_name)

The result is: No metrics graph data.

Is the idea to get metrics by using this expression totally wrong?