It can happen from time to time that a sync reports an error. There can be various reasons for such errors (file permissions are the most popular one). Unless the error is due to the source or the destination being disconnected, the sync will try to continue until the end.
A common source of errors are hidden system directories like “.DocumentRevisions-V100”, “.Spotlight-V100”, “.TemporaryItems” or “.Trashes”. These directories are usually located at the root of a device or in your home directory. If the error is happening on the source, it usually means that you’re trying to backup the entire filesystem (which includes system directories like /Library or /System that usually don’t hold personal data) or your entire home directory, but it is recommended that you only backup selected directories inside your home directory. If you still think that you need to backup directories containing one of those system items, you can either exclude hidden files or manually select the system items in the Exclude tab of the sync item’s settings. If, on the other side, the error is happening on the destination, it usually means that the destination is the root of an external device: in this case, it should help to create an empty directory and set that one as the destination.
Another source of errors can be missing permissions for files like your Photos library, Contacts, Calendar, Reminders and user directories like Desktop, Documents and Downloads. When trying to access one of these locations, you should get an alert asking for permission. In case you denied any of them, you can enable them at any time in the System Settings under Security and Privacy > Privacy. It is discouraged to sync the Library directory located in your home folder, because it contains data managed by macOS which often requires special permissions in order to be accessed. If necessary, you can add the Library directory to the Exclude tab of the sync item’s settings.
Two-way syncs can produce a size mismatch error when files with equal modification dates have different sizes; the problem here is that it cannot be determined which file should overwrite the other. “.DS_STORE” files can cause this error when the same folder inside the source and the destination is opened in two Finder windows, in which case they should probably be excluded from the sync (see Exclude files).
It is always recommended to have a look at the error log, which can be opened by clicking the error badge that automatically appears on the right side of the sync item or from the context menu. The error log provides information about which files caused the error. Particularly for two-way sync items, it is strongly recommended to sync again by carefully going through the confirmation dialog and making sure that no unique file is going to be copied or removed because of the previous error.