SharePoint - Embedding content from this website isn’t allowed
When embedding links or content to a SharePoint online site, the user may encounter the following message: "Embedding content from this website isn’t allowed, but your admin can change this setting. They will need to add ‘[URL domain address]‘ to the list of sites that are allowed."
Example:
Direct the customer to reach out to their site owner to have it enabled.
Have the site owner follow the steps here: https://kb.uwp.edu/page.php?id=119102