Please supply details of your operating system. What ver sion of lightroom is it an upgrade or full version.
My most likely guess is that you have downloaded the Lightroom Creative Cloud version and your Perpetual License serial number will not be able to activate it.
If this is the problem uninstall Lightroom, this process will not affect your Catalog, Preferences and personal files.
Download Lightroom from the link below and you license key should work fine.