Do you have the Lightroom 6 Standalone version.
If yes,Lightroom 6 comes with 30 day trial for Lightroom Mobile App and this message will not affect the Lightroom Desktop version.
You can also Sign Out from the Lightroom
Go to Preference
Click on Lightroom Mobile
Click on " Delete all Data"
Above steps will delete the sync previews to the Lightroom mobile App (If you have done that in the past)