Cannot install kube-prometheus-stack in k8s 1.15
Based on the kube-prometheus-stack repo, this helm chart is restricted for K8s versions 1.16.0 or above;
kubeVersion: ">=1.16.0-0"
Even though the github README says the prerequisites as Kubernetes 1.10+ with Beta APIs, internally the helm chart checks for the kube version to be 1.16.0 or above.
So I believe, you will need to try this on an upgrade K8s cluster.
If upgrading the cluster is not an option, maybe you could try the deprecated old version of this.
https://github.com/helm/charts/tree/master/stable/prometheus