If you experience issues using the the embed/built-in functionality, it could be because the iframe code has been removed.
Retrieve the code via "embed code" (from the Share icon/Embed icon) if possible and insert the code as "Source" in the text editor. Click 'OK' to save

Using the embed code allows users to view content within itslearning without having to go outside the platform. If you only include a link, such as a YouTube link, it will take students outside to the YouTube platform (which could be quite distracting for students)

You can embed content anywhere you see the itslearning rich text editor.