VM serial port connected to vSPC and VM serial port connected over network are not enabled on ESXi hosts in Compute cluster.
Resolution
Enable esxi firewall
In the webclient: Navigate to ESXi host > Manage > Settings > Security Profile > Firewall > Edit and check VM serial port connected to vSPC > OK Navigate to ESXi host > Manage > Settings > Security Profile > Firewall > Edit and check VM serial port connected to network > OK
In the HTML5 client: Navigate to ESXi host > Configure > Firewall > Edit and check VM serial port connected to vSPC and VM serial port connected to network > OK