You will need to contact Adobe licensing directly to resolve this issue.
I was not provided a serial number for the LR6 upgrade
I'm sure you think that's the case but I am certain that a version 6 upgrade serial number would have been emailed to you. It's standard practice for all perpetually licenced software sold via download. Perhaps it went to a wrong email address?
Have you tried signing it to your Adobe as account on their website to retrieve your version 6 serial number?
Thanks for the comments.
I expected to get the new serial # via email, and was surprised when I did not.
I ended up re-visiting my profile page and with several clicks in the correct places found the record of the download of the LR6 upgrade and along with it the LR6 serial number. My initial looks here had failed to turn up the serial #.
At any rate, now I am good to go!