If you are using Lightroom Classic you cannot simply "synchronize the catalog" unless your intention is to migrate the catalog and then switch to using Lightroom, the cloud-based version. If you choose that option then Lightroom becomes your primary controlling program and you no longer use Lightroom Classic, or you use it with a completely separate catalog and for a completely separate purpose.
If you want to continue using Lightroom Classic as your main program then you need to create collections, organizing your images as you want them to be seen in the cloud and then share those collections. The collections are what will be shared to the cloud.
So the question is, what is it that you are really wanting to do?