Under Target Details for Diagnostics Analysis you are unable to Select Target vCenter Servers.
This happens on the Skyline Health Diagnostics vCenter Plugin and not in the Skyline Diagnostics UI.
Skyline Health Diagnostics
This is expected behavior when Skyline Health Diagnostics is configured against multiple vCenters in Enhanced Linked Mode.
Skyline Health Diagnostics does not support Enhanced Linked Mode. If you would like to use the Skyline Health Diagnostics vCenter Plugin, you must have one Skyline Health Diagnostics Appliance for each vCenter. If you do not want to deploy multiple Skyline Health Diagnostics appliances the alternative is to use the Skyline Health Diagnostics UI where you can enter in any vCenter endpoint.