My environment:
- 1st VM/server: Active Directory Domain Controller
- 2nd VM/server: SharePoint 2016 On-Premises
- 3rd VM/server: Office Online Server (old Office Web Apps) also On-Premises - version from November 2017
Done already: Communication between SharePoint and OOS. I have also custom Office Excel Add-in uploaded to Apps for Office list in App Catalog site. Add-in itself should be fine - it works in other environment.
Issue: When I try to install add-in in Excel in web/browser, I'm getting "Your administrator has not installed Web Add-ins for your organization" message: screenshot
I have got also uploaded Office Excel Add-in to Apps for Office list in App Catalog, and it still does not work. Did I miss something in the configuration to allow adding custom add-ins?
I suppose a sharepoint catalog and app store add-ins are mixed in you post.
Instead of uploading the add-in Office 365, under Setting>Service & add-ins, try uploading the add-in in SharePoint Online Add Catalog site. To do this, please follow the steps below:
See Excel does not show Office 365 custom add-in for more information