Don't know how to sync data from my LR 6 on my older Mac into LR Classic, which I just installed

If you launch Lightroom Classic (and please be sure you are launching Lightroom Classic which has the LrC icon and NOT the new Lightroom which has the Lr icon), then you should be able to open the old Lr 6 catalog using File->Open Catalog and then select the Lr 6 catalog.

I installed the new LR Classic, but it did not sync with the photos in LR 6.

There is no such thing as "sync with the photos in LR 6". The photos will be available in the new Lightroom Classic catalog if you open it as described above, but this is not a "sync" in any meaningful sense of the word.

If you installed Lightroom Classic then Lightroom Classic should have upgraded the catalog. That upgraded catalog is what will link and synchronize to the cloud. But you cannot just synchronize the images as such. It will be necessary for you to create "standard" collections and then synchronize those collections. It is the collections that get synchronized, not every image in your catalog. Please note that it is only possible to synchronize a single catalog at any given time.

What data are you expecting to be uploaded to the cloud? If you are using Lightroom Classic it's necessary to create collections and synchronize those collections to have smart previews synchronized to the cloud. Images don't just automatically get sent to the cloud because you are using Lightroom Classic.

This message means you need use your operating system to find the catalog file you are trying to open, and in that folder there is a file whose name ends with .LOCK. Delete this .LOCK file. DO NOT DELETE ANYTHING ELSE.