My environment:
SharePoint 2016 + Office Online Server
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:

Did I miss something in the configuration to allow adding custom add-ins?