If a document, worksheet, or presentation is locked for editing and can’t be opened, it may be that the file is already being edited by someone else, or they have the file checked out. Let's take a quick look at some of the reasons why you might not be able to edit the file.
Another author has checked out the file
If you open a file stored on a SharePoint site, more than one person can work on it at the same time. But if you check out the file before you open it, other authors are locked out of it until you check it back in.
If required check-out is turned on for the document library in which the file is stored, the co-authoring features aren’t available.
Another author is using a different version of the Office application
If an author opens the document in a previous version of the Office application, the co-authoring functionality isn’t available. For example, if the file has been opened in Word or Excel 2010 then co-authoring isn't available.
The server doesn’t support the co-authoring functionality
The co-authoring functionality is available when a document is saved on a SharePoint Foundation site that supports Workspaces, on SharePoint Online, or on OneDrive. If the file is saved on some other kind of server, however, it's possible that the server it's stored on doesn't support co-authoring.
It’s also possible that your organization’s policy administrator has disabled the co-authoring functionality on either the server or on your computer.
The file format isn’t compatible with the co-authoring functionality
The co-authoring functionality can be used with OpenXML files (such as .docx or .xlsx). The co-authoring functionality is not available for .doc files (the Microsoft Word 93-2007 format), .rtf files, or other formats.
You don't have edit permissions for this file
If you do not have edit permission for the file, the co-authoring functionality is not available.
Other features that can’t be used with the co-authoring functionality
The co-authoring functionality is not available if the document is marked as final or if it includes the following:
Information Rights Management or Digital Rights Management
The co-authoring functionality is not available if the document is a master document or a subdocument, or if the Store random numbers to improve Combine accuracy option is not selected.