While syncing images in Lightroom, one or more images remain in syncing state forever. Every time you launch Lightroom, you see that the same number of images are syncing and you do not know which images are stuck in syncing.
Click the filter icon at the top and select Sync Status > Syncing.
Lightroom now displays those images that are trying to sync.
Select all the images and do one of the following:
If filtering the images based on sync status returns no result, then chances are that the photo which is stuck in syncing state is in the Deleted folder.