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 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".
This error messages (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 before you tried to save it back to the server.
For the sake of document integrity you can only upload the exact version that you downloaded – and only if it is checked out by you.
Because 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 continue to have problems please contact support.