How do I delete a default AKS node pool when I get an error saying it's the "backend address pool of the basic load balancer"
If my assumption is correct that's related to having a loadbalancer that wasn't correctly deleted.
Which could be resulted by redeploying a service (of type: load balancer) in a short time before Azure cleans up the previously associated loadbalancer resource.
To my knowledge, that could be fixed with the rolling of Kubernetes 1.17 and the Loadbalancer Finalizer feature, through this pull.
please update the question if that showed up with the new cluster version.