Cannot start dataproc cluster because network is not found

When I run

hailctl dataproc start my-cluster

I get this error:

(gcloud.dataproc.clusters.create) NOT_FOUND: The resource 'projects/my-gcp-project/global/networks/default' was not found

This means that your Google Cloud project is not configured with a default network. When you have this kind of Google Cloud project, you must explicitly specify a network with --network NAME:

hailctl dataproc start my-cluster --network NAME

You can see the available networks with

gcloud compute networks list