Moderator: Moved to vSphere Community as Skyline Health (vSphere Health) is part of vCenter and is not supported by the Skyline Community.
I think your vcenter can't access to ESXI Management and return this error.
check your connectivity between vcenter and ESXI
We were in a similar situation.
We have 2 linked vCenters 6.7 U3, VC1 (which it's been there since v6.0 or before) in HA, and VC2 (added ~1 year ago) on a different site.
Up until the latest patching, the tab was visible and working on VC1, but we were getting the same error on VC2. Now, if I connect to VC1, the Skyline tab is gone for both vCenters. If I connect to VC2, it's there as it was before (working on VC1, error on VC2)
I checked the vsphere_client_virgo logs, and I could see a bunch of errors like "Server certificate chain is not trusted and thumbprint doesn't match" referring to VC2. Indeed, the thumbprints were not matching.
I followed this KB https://kb.vmware.com/s/article/2121689 on VC2, and I can now see the Skyline tab on VC2 (if connected to VC2). This should solve your issue, if you have the same messages on the logs.
I still can't see the tab at all if connected to VC1, but I think that's a different issue.