When you try to deploy prometheus operator helm chart. You may find the following error:
Error: failed to install CRD crds/crd-alertmanagerconfigs.yaml: unable to recognize "": no matches for kind "CustomResourceDefinition" in version "apiextensions.k8s.io/v1"
So basically helm is complaining about no specific CRD under apiextensions.k8s.io/v1. The solution is actually pretty straight forward. The Kubernetes version I was working with is too low.
In this case, I was working with Kubernetes 1.15. I need to upgrade to 1.16 to make it work.