Please check if you are signed in to Lightroom mobile as well.
Open Edit>>Preferences>>Lightroom mobile
Ensure that you are signed in.
If signed in and subscribed but issue remains the same.Then send us the Diagnostics log(via Private message)
You can trigger the log via Lightroom Desktop Edit>>Preferences -> Lightroom Mobile and when you hold down the Alt/Option key you will notice a generate diagnostic log button.
Do you have more than one catalog? It is only possible to sync one single desktop catalog although you can sync with multiple devices.
I only have the one desktop catalog that I use.
Is the sync icon showing to the left of the collection name or are you saying it is not possible to mark the collection for sync? When you select a correctly marked collection each thumbnail should also show the sync icon with a button top-right “Make Public” which is an additional option.
The other thing to check is that sync is not paused. Hover your mouse over the nameplate top-left and click the drop-down icon that appears.
Sync wasn't paused, and it didn't have the sync icon next to the collections. Also, when I clicked "Create Collection" it wasn't even giving me an option to sync with lightroom mobile.
Are you running the latest version of Lightroom CC?
I suggest launching Creative Cloud. Then go to the Apps tab and check to see if any updates are available.
Everything is up to date, but it did randomly start working again. I don't know if it was a connection issue or what, but now the Sync feature is back on under "Create Collections" and the icons are back next to the collections!
I had the same problem as you, sync with mobile disappeared although the tutorials online show the sync is there i think those are out of date now and need updating. I saw this post and saved it. Basically the sync is more web based now and controlled from a website lightroom.adobe.com I use safari so I guess the other web browsers work the same way, you login in on your cc account. Once i got in here it showed me what sync collections I had set up and you can manage things from within here, I bookmarked this page and it seams you have to have this page open all the time so it can sync to all your other devises once your happy you can close it and open the bookmark at a later date. So anyway with the web site open now open the latest version of lightroom. i spotted this tutorial which was more helpful. Edit and organize photos on mobile or desktop | Adobe Photoshop Lightroom CC tutorials
Top left i noticed LR lightroom drop down and found sync with lightroom mobile and started this. I copied my collections from old sync because it asked me to delete the old collections for some reason. So having figured things out a little i noticed I now have a boxes available next to all my collections which i can tick to sync any number of collections to mobile. On the mobile apps you have to login to your cc account. I got my wife to download the app onto her phone and logged in on my account to see if she could see the synced collection and she can, which is a great. I'm sure there are sharing options but I don't need them. The only thing for me now is I have various catalogs and I hope you can repeat the process in different catalogs with different collections.
Hope this is more incitefull.