@MattStOnge The error you're seeing is expected. Anytime you open an Office file from SharePoint (any version, SharePoint 2010, 2013, 2016, 2019, Online, etc) using Office (2010, 2013, 2016, 2019, 365 Pro Plus, Office for Mac, etc), you are guaranteed to set a file lock. That's how the product works.
There are two types of locks that can be set, and Exclusive file lock, and a Co-authoring file lock. Which lock type is set is dependent on a few key factors. When opening Office files, SharePoint will always attempt to set a co-authoring lock first. If a co-authoring lock cannot be set, SharePoint falls back to an exclusive file lock.
Exclusive file locks act exactly as their name implies. User opens the file, because of certain factors (addressed below), an exclusive file lock is set by the user's session in SharePoint. As long as that lock is in place only the user who set the lock can edit the file. If the user saves the file when they're done and properly exits the file (doesn't put the PC in sleep mode with the file open, doesn't just close the laptop lid and walk away, doesn't leave the file open for hours on end, and instead either uses file > Close, or waits for the file to completely save, then closes out of the client), then Office can send the proper network traffic to indicate it's done with the file and SharePoint will release the lock. This is known as exiting gracefully from the file, and applies to both file lock types.
Co-Authoring file locks are applied when the file is in a state that allows co-authoring and if the user is in a client that supports co-authoring. Whenever a file is opened in Office Web Apps, Office Online Server, or Office for the web, where the file can be edited in the browser, a Co-authoring file lock will always be set.
Certain Office clients (installed on your Windows or Mac machines) do not fully support Co-Authoring. For example, Co-Authoring in Excel is only supported on Office 365 Pro Plus version 1808 and newer. All other Office desktop clients do not allow for Excel co-authoring, and will always try to set an exclusive file lock.
Outside of the Office clients, there are other factors specific to the file and where the files stored that determine what file lock type is set. For example, all of the following to cause only Exclusive file locks to be set:
- The SharePoint library where the files are stored has "Checked required for editing" enabled. Even if opened in the browser, only exclusive file locks can be set for files within such a document library.
- IRM controls on the SharePoint library or the file.
- Setting the registry key "fsshttpoff" to a value of 1 (this is NEVER recommended)
- Turning off the OneDrive Sync client "Use Office applications to sync Office files that I open" setting (this should be left 'On' by default).
- Using the legacy Office file formats (*.doc, *.xls, *.ppt) instead of the current XML based file formats (*.xlsx, *.docx, *.pptx)
- Syncing files with the old and deprecated groove.exe OneDrive for Business sync client.
- And a few other, rarer factors.
@MattStOnge, for you, it sounds like other users have set a co-authoring lock on the file, but your machine can only set exclusive file locks. Since the file is Excel, check that your Office client actually supports Excel Co-authoring. If you're not on the Office 365 Pro Plus client on any of the current update channel builds (1902, 1908, 2002, 2003, 2004) then you're never expected to be able to edit the Excel file in the excel client app in near real time with your colleagues. I would compare what version of Office you're using with that of your colleagues. If it's the same version, then your machine has another factor at play preventing you from joining the Excel file's Co-authoring session.
This functionality is more than just SharePoint or Office, there is a lot more in play. What settings are set in the SharePoint Document library, on the file itself, in the Office client, on your windows/mac machine, what group policy settings are set (if your local user account and machine are domain managed), all of these matter and can affect what type of file lock your Office client can set, and by direct relation, what can be done with said file.
