"Your administrator has not installed Web Add-ins for your organization" when try to add Office Add-in for SharePoint 2016 + Office Online Server

Question

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?

Answer 1

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:

1. Sign in to Office 365 portal with admin credentials.
2. Go to Admin Center>SharePoint Online Admin Center.
3. Once you are in SharePoint Admin center, go to App Catalog site, URL should be something like https://tenant.sharepoint.com/sites/AppCatalog
4. Click Apps for Office on the left navigation panel to upload the add-in for Office apps.
5. Click Upload and browser the path where the custom add-in is saved on your system.

See Excel does not show Office 365 custom add-in for more information