I reinstalled Illustrator CC and for some reason the sync button on the bottom left is greyed out and so is Edit>Sync Settings. I am signed into Creative Cloud, so I have no IDEA what the deal is!!!!
1. Have you signed-in to https://creative.adobe.com/ once?
2. If yes, please try clearing Ai preferences once and then check if it gets enabled.
3. You can also sign-out once, relaunch Ai and then sign-in again and then relaunch again to see if it works.
4. Please check if there is no firewall in your computer that might be blocking the creative cloud connection at the backend.
Please let me know if any of these works.