Ensure the file is not currently checked out by another user. If so, the file will have to be checked in. Files can only be checked out to one user at a time. If the file is checked out by another user, a request can be sent to them by right-clicking on the document and selecting ‘Send Check-in Request’. This will prompt the user via email that the file is required. If the file is not checked out, the file is likely to be in a workflow state that does not allow editing (read-only), move the file to an editable workflow state to work on the file.