The error stating "Prism Central is unreachable" when trying to add a cluster to an existing Prism Central with a proxy setup is likely due to incorrect or missing network routing configurations or firewall rules that do not allow communication between Prism Element and Prism Central. The most likely configuration that is missing and needed to resolve this issue is the whitelist of Prism Central VIP inside of Prism Element and Prism Element VIP inside of Prism Central. Whitelisting these Virtual IP addresses ensures that network traffic between these crucial components of the Nutanix architecture is permitted, thus enabling successful communication and registration of the cluster with Prism Central.References: Nutanix Network Configuration Guide, Nutanix Prism Central Administration.