This likely means you tried to pass in a Kubernetes version that Kubespray (or your current Kubespray checkout) does not yet support. Try pulling the newest release of kubespray and try again.
If the SLATE registration playbook throws this error, it means that the ingress controller was unable to get an IP from MetalLB in time. Log in to one of the controller nodes in the cluster, use sudo /usr/local/bin/kubectl get pods -n metallb-system to get the name of the MetalLB controller pod, and then check its logs with sudo /usr/local/bin/kubectl logs controller-... -n metallb-system.
If the logs say something related to “Invalid CIDR”, it likely means the CIDR range you passed into the MetalLB Configuration was invalid (e.g. forgot to append /32 to the end of a single IP). Try adjusting the CIDR range and rerunning the Kubespray playbook to see if this fixes the issue. Else, we suggest pinging the SLATE Slack for more help.