If you registered LR with Adobe then you can find your registered serial number by logging into adobe.com then clicking Manage Account then Products and Services then View All Products.
That's what I did. I typed in the serial number, and it won't accept it...
The serial number you’re trying to use doesn’t match the version of LR you’re installing or Adobe has blacklisted the serial number as being stolen or pirated, or you’re typing it in wrong. How about copy/pasting it from the Adobe.com registration page into LR just to make sure you’re not copying something wrong. Where are you getting LR from that you’ve installed? There is a standalone-serial-number-licensed version and a CC-login-licensed version. Do you have a subscription to CC Photographer’s Bundle?
I figured out the problem.
I downloaded the regular version of Lightroom, but I should of been downloading the student and teacher edition.
I downloaded the student and teacher edition and entered the serial number, and it worked.
Thanks for your help!