Hi,
The easiest way is to create an image custom field and to use it as featured image.
Or, you can refer to this topic https://support.advancedcustomfields.com/forums/topic/youtube-thumbnail-object-with-oembed-field/
Hi,
It may be a stupid question but did you fill your custom fields in the post/page you want them to be ? Because your code is correct so it could be because of that.