Hello there,
Windows 10 in S Mode is not compatible with the full version of Office 365.
As far as I know, Windows 10 S Mode only allows for installing Microsoft-verified apps from the Microsoft Store and requires Microsoft Edge for safe browsing. Could Office 365 (Version 1808) be installed smoothly on your PCs?
Switching out of S mode in Windows https://support.microsoft.com/en-us/windows/switching-out-of-s-mode-in-windows-4f56d9be-99ec-6983-119f-031bfb28a307
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue.
Hope this resolves your Query !!
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