'root' login is only for the vCSA operating system. It can be used to login to the appliance OS and the VAMI page. To login to the vCenter (via Web Client or HTML), you need to use the vCenter application account. By default, you can use administrator@vsphere.local account. For this account, you would have set the SSO password during the time of installation.
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Cheers,
Supreet
