Hi Keith,
How do you set the “require check-out/check-in” setting? Have you set it via LIBRARY>Library Settings>Version Settings>Require Check Out>Yes? If yes, I’d like to explain that the yellow message letting users select Check Out in a file before editing it will only appear in Excel not Excel Online. Please confirm if you see the yellow message in Excel. If you get the error messages in Excel Online, it is a normal behavior. You need to check out the file first and then edit it in Excel Online. If you see the yellow message in Excel Online, could you give us some related screenshots?
If you want to check out multiple files once, I suggest you use the Site Content and Structure feature.
You need to active SharePoint Server Publishing Infrastructure in Site collection features under Site Collection Administration to enable the feature.
After activing SharePoint Server Publishing Infrastructure, you will see the feature available under Site Administration.
Then you can check out multiple files via Site Content and Structure>Select the library where files you want to check out are>Select the files once>Drop down Actions>Click Check Out.
Please let us know your updates at your convenience.
Thanks,
Cliff