vCenter 5.5 SSO Appliance?
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Unable to detect appliance type. If this is a vCenter 5.5 SSO appliance, to proceed with the upgrade click Yes. Otherwise, click No to go back to the previous screen.
To resolve this issue, change the root password of the PSC, and correct any issues seen with the password expiry (such as if it is in the past).
The upgrade process should now complete without issue.