Office add-ins may not work if your version of Office is running on Windows 10 in S mode. The solution is to switch out of Windows 10 in S mode.
Additionally, even after switching out of Windows 10 in S mode, some add-ins may still not work if you have a Microsoft Store install of Office. To fix this, you need to switch to a Click-to-run install.
Check your operating system
If you're not sure what operating system you have, see Which version of Windows am I running?
Check the type of Office install you have
If you're not sure what type of install you have, do the following:
Create or open an existing Office file and select File > Account (or it might say Office Account).
Under Product information, find the About button and check for the following:
Click-to-Run installations have a Version and Build number, and include the phrase Click-to-Run.
An MSI installation doesn't have a Version or Build number.
Microsoft Store installations have a Version and Build number, and include the phrase Microsoft Store.
Fix your add-ins
If you're running Windows 10 in S mode you need to switch to Windows 10 Pro. See Switch out of S mode in Windows 10.
When this is complete you can check if your add-ins are working with your installed version of Office, or go ahead and uninstall the Microsoft Store version and then reinstall Office using the Click-to-run version.
Uninstall the Microsoft Store installation type of Office.
Install the Click-to-run version of Office.
Tip: ITPros who manage enterprise versions of Office should follow the installation guidance in Deployment guide for Office 365 ProPlus.
Once the Click-to-run install is complete, go ahead and install and activate your Office add-ins.
For IT departments only
The following section is only for IT departments running Windows 10 in S mode on devices in an enterprise or in a school.
Office in enterprises
Businesses are advised to upgrade their Windows 10 in S mode devices to Windows 10 Pro, then install a business edition of Office.
Office in schools
If your school doesn't have any Office add-in requirements, you can continue to run Windows 10 in S mode on a device. Your school needs both an Office 365 tenant and a Microsoft Store for Education tenant. Once you have purchased an Office 365 education plan, your IT Admin may then install this suite using one of the following methods:
Install via the Set Up School PCs app (Recommended)
This app is available in the Microsoft Store for Education and allows the IT Admin to create a deployment package for the Office bundle and other apps. The deployment package may be downloaded and copied onto a USB key for repeated deployment across devices, minimizing network bandwidth usage. This approach is recommended for bulk deployment scenarios. IT administrators can download the app from the Microsoft Store at Set Up Schools PCs.
For more information setting up and using this app, see Use the Set up School PCs app.
Install via Intune
IT Admins may also choose to push the Office bundle to each device via Intune for Education. This means that Office must be downloaded and installed via the network each time, using up more network bandwidth. This approach is recommended for one-off deployment scenarios.
For more information about using Intune for Education, see How do I add apps to Intune for Education?
After your IT Admin has installed Office on your Windows 10 in S mode device, anyone at your school who wants to use Office on that device can do so if they have been assigned an Office 365 "seat" that includes Office client rights. Office client rights are included in any of the following Office 365 subscriptions:
*These plans are available as part of the Teacher Use Benefit and Student Use Benefit programs.
In many cases, all students at a school will have access to Office client rights via the Student Use Benefit Program. After Office is installed, students and faculty assigned an Office 365 seat with client rights can simply sign in with their Org ID and use those Office applications on Windows 10 in S mode.