Is this a managed Symantec encryption desktop? Can you check if the SSO policy is enabled from Encryption server?
Or the SSO policy was enabled after the disk encryption was successfully done on this user system, There may be possibility that policy isn’t updated on the Encryption desktop. A re-enrollment with encryption server will help in this situation.
If this is Standalone encryption desktop, add another passphrase user to the disk as backup, Delete and add the exsiting user with SSO check box enabled.