while installing lr6, make sure you're accurately entering your lr6 serial number first and when prompted, enter your qualifying product (lr5) serial number.  it's essential that you enter the correct number at the correct time.

if you're certain you're doing that accurately and still see that revoked message, contact adobe support to get clarification.

there are 2 ways to contact adobe; chat and twitter:

chat:
use a browser that allows popups and cookies
and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true