Actually LR Classic does Not Sync with LR CC Mobile.
LR Classic syncs Smart Previews of images placed in a collection that is synced with the WEB based system and then from there it syncs down to LR CC mobile.
If you are asking about the Desktop version of LR CC then that 2 doesn't sync directly to LR CC Mobile or LR Classic. It all goes through the WEB system first.
If you would explain the exact program you are using on your computer and the exact problem and or error you are getting we can try to help you.