Unable to load add-in with Word/Office Online Server

Question

I have completed the development of an add-in for Word, but I am having troubles getting it to load in a Word Web App using the Office Online Server. The error I get says:
Add-in Warning This add-in is not available in Office Online. Open this document in desktop version of Office to use the add-in.

I have tested this and it does run correctly with both the Word Desktop app (MS 365) and with Word on the Web via SharePoint/MS 365.
I am also using OpenXML to add the necessary tags to allow this to auto-open when the document is loaded. And using the FileShare option to indicate where the manifest is located.

I have been searching and have found no information on what might cause this warning.

I have also used the npm office-addin-manifest validator, and it found no issues with the manifest. It also stated that it supports (among other things) Word on the Web.

If anyone has any insight as to what the issue is, it would be appreciated.

Answer 1

This isn't about loading 3rd party add-ins. This is an add-in that I have developed. The issue stands around how to deploy it for Office Online Server. The add-in works perfectly fine when deployed to Windows 365 (desktop or web app), but fails when loading with Word in Office Online Server. Why is this failing when it works with the other versions of Word?