Welcome to communities,
As per the information shared, We see that you are trying to register vCenter Server with Skyline using the IP Address.
Request you to use FQDN to add vCenter server.
If you have customer SSO Configuration in your environment for vCenter like External PSC or Custom Domain other than vsphere.local
Please use Customer SSO configuration details.
Same error when using FQDN. I do not use custom SSO
Check if you are able to login to vCenter using the email address which is used to configure vCenter endpoint.
If you are still receiving the same error.
Request you to share the contact details using the Smart Sheet, So we could contact you over the phone and take a webex