After creating a collection you can sync it to the cloud, once its synced then you can access it on your IMac via Lightroom Web Link: Adobe Photoshop Lightroom.
For Mobile Device you need to install Lightroom Mobile (IOS & Android) . Refer: Get started with Lightroom on mobile | Adobe Photoshop Lightroom CC tutorials
Let us know if that helps.
I can see my Collections on my Moblie Devices but not my Collections sets.
I have made Collections called Turkey, Greece, Spain and more and a Collections set called Vacations in Lightroom on my computer.
Will I get an answer from Adobe or what is the ”next step”?
You would only be able to see the collections of the collections sets.
Collection sets can not be synced only the collection in it can be synced and will be visible to you on your mobile device.