"Sync" problems are the most frequent questions in the (Cloud) forums. A Googly search gave 1,580,000 results 🙂
Very few people seem to have definitive answers, and I am not one the them 😞 My 'sync' just worked without problems.
Your Lr-Classic knows there are files to Sync 'down' but is having trouble completing the download for some reason. Does Lightroom have full permissions in Mac System Preferences? (especially if using Catalina OS).
Some suggest resetting the Sync data-
MENU: Edit > Preferences > Lightroom Sync tab > hold down the Alt/Option key to- [Rebuild Sync Data] (The Cog wheel on the Cloud icon dialog will also open the Sync Preferences.)
Here is a youtubi link that suggests 'Files deleted in the Cloud are the problem'-