A user recently ran into the following error when attempting to embed a Microsoft Stream video on a SharePoint 2016 site:
“The embed code is invalid because the source of the embed content is not allowed.”
To resolve this, you must be a site collection administrator.
Go to site settings and select “HTML Field Security” under “Site Collection Administration.”
By default, SharePoint allows embeds from YouTube, Bing, Vimeo, and Microsoft but we need to add web.microsoftstream.com to the list.
Now try to embed your video again, and it will work.