There are two different versions of Lightroom now. Lightroom Classic is the one that functions similarly to Lightroom 5 in that it utilizes a local catalog and stores images locally on your computer hard drive. Lightroom is part of the new ecosystem, and stores your images in the cloud. If you installed Lightroom (not Classic) and that is your intention, then you will need to install the trial version of Lightroom Classic long enough to enable that version to upgrade your catalog. The upgrade process will create a new copy of your catalog that will be compatible with the new cloud version, and then it will be possible for you to migrate the images to the cloud.
If your intention was not to install the cloud version of Lightroom then you should have installed Lightroom Classic. The first time you run LrC it would have prompted you to upgrade your existing catalog which would have automatically created a new copy of your catalog. So the question is, what were your intentions when you chose to switch to the newer Lightroom?