The Lightroom 6 catalog is not compatible with the Lightroom cloud version and will not migrate or export. At the very least you would have to install the trial version of Lightroom Classic long enough to enable Lightroom Classic to create an upgraded copy of your catalog. This will not replace your Lightroom 6 catalog, but will create a new copy that would be compatible with the cloud version. Then that catalog could be migrated to the cloud. You could then uninstall Lightroom Classic. Be aware, however, that there would be no interaction or compatibility between Lightroom Cloud and Lightroom 6, and there would be no interchangeability of images between the two versions. You would have to decide which version you would want to edit your images with. The better choice, in my opinion, would be to upgrade to the photography package which would give you both Lightroom Classic AND the cloud version as well as Photoshop.