If a document, worksheet, or presentation is locked for editing and can’t be edited in the Windows desktop applications, it may be that the file is already being edited by someone else, or they have the file checked out.
Not sure what "checked out" means? This article might help: Check out or check in files in a document library.
Let's take a quick look at some of the reasons why you might not be able to edit the file.
The file is locked due to a previous crash
If you had a crash with the file open previously, it's possible that the application still has it locked. This is usually the cause when you open a file and it says that the file is read-only because you already have it open.
One simple thing to try is exiting the software normally, then rebooting your computer, which should cause the lock to be released.
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. Ask them to save and close the file as soon as they're finished in order to unlock the file for you.
This also applies to you, of course, make sure you're using the current version of the Office application.
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.
If any of those are the case, and somebody else has the file open, you won't be able to edit until they close it.
The file format isn’t compatible with the co-authoring functionality
The co-authoring functionality can be used with Open XML files (such as .docx or .xlsx). The co-authoring functionality is not available for .doc files (the Microsoft Word 93-2007 format), .rtf files, templates (including modern format .dotx, .potx), or other formats. It's possible the file is in one of those unsupported formats.
Depending upon the format your file is in, the easiest solution may be to save a new copy of the file, in the modern format, and work from that new copy.
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. Ask the owner of the file to grant you edit permissions.
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 in File > Options > Trust Center > Trust Center Settings > Privacy Options.