This error usually means that either the user or someone else still has that file open. This can happen if the file referenced is open in another program or if a crash occurs while uploading. To resolve it, first make sure that no users have the file open anywhere, then reboot the machine to make sure it is not open as a remnant from a crash. If the same error comes up after rebooting, contact OCLC Support and send them the collection name, project name and a screenshot of the error.