You are still using the creative cloud installation of Lightroom. It cannot be activated with a serial number. To solve the problem you will need to uninstall Lightroom and then install from this website:
If you are not in the United States, it might be necessary for you to scroll to the bottom of the screen. In the left-hand corner indicates that the region is the United States, but there is a change prompt as well. You should be was to change to a region that will allow you to download the update. You only need Lightroom 5.7.1, which was the final update that was released for Lightroom 5.