This happens for two common reasons:
There’s a typo in the URL – even a small mistake will cause the preview to fail.
The website blocks iframe embedding – some sites prevent their content from opening in a popup or inline frame.
How to Fix It
Check the link
Carefully retype the URL in the Hyperlink or Embed field.
Make sure the domain and path are correct.
Switch to open in a new tab
If the site does not allow embedding, turn off Open in popup.
The link will then open in a new browser tab, which always works.
If you own the website
You can whitelist Storydoc’s domain to allow iframe embedding.
Where This Can Occur
Hyperlinks set to Open in popup on buttons or text.
Websites added as iframes inside your story.