This one:
"Centrify.Cloud.Core.ProxyNotFoundException: Unable to connect to a Connector to service this request."
Means that there are no connectors available for that network to satisfy the request.
This depends on the service in question. Let's say this was to be able to proxy an RDP session for a specific subnet. That means that there are no connectors for that subnet to pickup that workload.
"Failed to connect to address net.tcp://<servername>:9521/OnPremRpc with spn null and impersonated user null."
This one could mean that when the system queried Active Directory via service principal name lookup, it could not find an entry or that particular site. The protocol was going to be net.tcp and there's a binding over port 9521. It could also be a firewall in between.
Without additional context it's hard to make up the whole meaning.