Hi! I bought Creative Cloud Photography plan (one-year, prepaid)! I have been downloaded and setup creative cloud, authorization with my adobe id, updated my Lightroom 5.7 to 6 CC and now i'm watching: you have trial version for 30 days and i need serial number. Where is this serial number ? I payed for what ?
Second, check that is shows as Lightroom CC and not Lightroom 6. (See the difference here, but do not follow this guide since you want CC version)
I have the same problem with Lightroom 6 (stand-alone). Every time I restart my machine, and periodically when I restart LR (so I leave it open now and don't close it), I'm prompted with the same thing, and have to re-enter my serial number. Heck, I almost have the dang thing memorized now!! C'mon Adobe, hurry up with the bug-fixing revision--this isn't the only problem!