When I tried to create a Collection in Lightroom Classic, in the Create Collection dialog, I do not have a Sync With Lightroom check box.
How can I get this check box to display?
Did you enable sync for this catalog? You only get that option if sync is already enabled.
Additionally, synchronization can only be enabled from one catalog. If you have opened another catalog that is NOT the catalog that is synchronized with Lightroom then you will not be able to synchronize any images from that catalog.
True, but with a remark. You can enable sync in any catalog. What will happen if you do so but have already synced another catalog, is that Lightroom will ask you if you want to disable the sync of that other catalog and continue with this one.
I suppose I look at it a little more simplistically. If I open a catalog other than the one that isn't synchronized then the boxes to check to synchronize are not there. If there is something else to consider then I'm not aware of what that might be. At least that's the way things work on my Windows 10 computer. I'm not a student on the online help system. So there is a lot of assumption in the answer I provided.
The boxes aren't there if the catalog isn't synced. The reason could be that another catalog is synced, like you said. It can also be that no catalog is synced yet.
I don't want to belabor this point. But that was the reason for my answer in the first place. Since only one catalog can be synchronized, on my computer at least, sync boxes appear in the catalog that IS synchronized, sync boxes DO NOT appear in other catalogs. That is what I have experienced, and that was the reason for my response. If that isn't the way it is supposed to work then I am willing to stand corrected.
Yes, you are completely correct. That is what I am saying too.