For Mac users this error message can be read: "Error: The chapter is not checked out, and can't be uploaded to the server".
For Windows users this error message can be read: "Error: The document is not checked out, and cannot be saved to the server" or "Failed to verify the token for upload: HTTP 401".
The error message seen above occurs if you are attempting to upload a document that is no longer checked out by you. Often because the chapter was unlocked (by you or by an Administrator) before you saved it back to the server. Since the document is not checked out, you will not be able to save it back to the server.
To work on the file, you will need to download it again for editing. If you have already made changes in the document, you can copy the changes from this version and paste it in the version you have redownloaded from the server. If you continue to have problems please contact support.