Try these solutions in order:

Step 1: Make sure that the OneDrive for Business sync app is up-to-date

Step 2: Make sure that your file and folder names don't contain unsupported characters or invalid file types

If a file or folder name contains certain characters or file types, you won’t be able to sync those files. Review the list of invalid characters and file types, and change any file or folder names, or remove the file types that aren’t supported.

Step 3: Make sure that your file sizes, items counts, and file path lengths are within the limits

If a file you’re trying to sync is too large, or if you’re trying to sync too many items, OneDrive for Business sync might not work. Review the limits for the size and number of files that can be synced, and make sure the files you’re trying to sync don’t fall within the listed restrictions or limitations.

Step 4: Repair a OneDrive for Business sync connection

Follow these quick steps to repair a sync connection with OneDrive for Business, and see if that fixes your sync problem.

Step 5: Stop the sync and restart

If you’re having problems with a synced library folder, it’s sometimes simplest to just stop syncing the library, and then start syncing the library again, as if for the first time.

Step 6: Use the OneDrive for Business Sync Issues Troubleshooting Guide

If you’re still experiencing sync problems, try the OneDrive for Business Sync Issues Troubleshooting Guide. This tool can help guide you through diagnosing and resolving issues with libraries you’re currently syncing.