Twitter widgets rely on a JavaScript file loaded from platform.twitter.com/widgets.js .
The widget preview page should show a preview of your configured widget and a textarea with HTML code.
https://twitter.com/settings/widgets
If you are not seeing a preview on twitter.com or the rendered widget on your website it’s possible your browser is blocking loading Twitter’s JavaScript. Try disabling browser extensions such as ad blockers which may interfere with loading Twitter’s widgets JavaScript.