Did you download the Creative Cloud trail version of Lightroom, or the stand-alone version of Lightroom? You most likely need the stand-alone version
As dj_paige says you probably have the trial of the Creative Cloud version installed (this version cannot be activated with the perpetual license key). Uninstall the one presently on your computer this will not affect your existing catalog, preferences and other personal files. Then download LR 5.6 from the link below, install and insert your serial number and it should work fine.