Using LR cc, when I create a new standard collection there is no check box for sync to LR mobile.
I have tried deleting all data in the LR mobile tag in Preferences, re starting LR and signing out of my CC account.
If you're not syncing with Lightroom mobile, the sync checkboxes won't show up in the dialog that comes up when you create a new collection or next to the collection names in the left panel.
If you're signed in to Creative Cloud in Lightroom, your user name should show up in the upper left corner of the screen. Click your user name and you'll see a drop down with a few options. The first one should say "Sync with Lightroom mobile" and to the right of that it'll probably display the word "Start". Click "Start" to start the sync. I'm not 100% sure, but you might need to restart Lightroom too, but that should get those checkboxes to show up.
I have the same problem, no check box against any of my collections to sync them with LR Mobile. I am signed into my LR CC account and have clicked the "Sync with Lightroom mobile" start and restarted LR but still don't see any boxes to select which get sync'd.