Migrating a catalog from old computer to new computer where images are stored on EHD

When you say you were using 'Lightroom 6.5', do you mean the years old predecessor of Lightroom Classic, or last years version of Lightroom 7.5? If the former, then you need to upgrade that catalog first before you can migrate it to Lightroom 7.5. Install a trial version of Lightroom Classic, and open the Lightroom 6 catalog with it. Let it upgrade the catalog and then that upgraded copy should migrate just fine. You may want to consider using Lightroom Classic on that new computer in this case, because that is the latest version of the Lightroom you know...

I'm not sure what you mean when you say that you cannot load any pictures or catalog system on the new system. You should simply be able to log into your Adobe account using the CC Desktop app on the new system, start Lightroom Desktop and your local catalog will automatically be recreated using your master cloud-based catalog. Assuming your cloud syncing was fully up-to-date on the old system, you should have no trouble setting up the app on the new system.