The only trial version is the creative cloud version, which does not require a serial number. If you decide to purchase Lightroom 6 then there is a process involved to convert the creative cloud version to the standalone version. So, what did you actually download?
If you have download Lightroom 6 then it would ask you for the serial number, and you can ignore it and still use it for 30 days as a trial and need the key for activation after it.
If you go with Creative Cloud Subscription, you need to subscribe for it Lightroom CC and need your subscribed email id to activate it.