Sync errors can occur for several reasons with OneDrive for Business. The first step to take it to test internet network upload speed. This is easily done by going to www.speedtest.net. If the upload speed is not within 30% of the internet subscription max the local router, hub, etc. should be checked and the internet provider should be contacted.
Another simple but effective solution is to restart the computer. This creates a new connection to the internet and to OneDrive for Business and will often resolve the issue.
If multiply files or larger files to selected to sync, it is recommended that the upload button be utilized. Drag and drop works best for small batches of files and small files.
The next step is to repair sync connection in OneDrive for Business. For Microsoft's latest instructions on repairing the sync connection and other sync issues, click here.