Why won't my preview display an email with a product feed?
If the email you are trying to preview has never been sent before (i.e. a flow email that's set to Draft status), the product feed is not yet associated with that message. Once you send the email for the first time, your previews will display normally. If you're having this issue. you can test to see if it is your product block by removing the product block and trying to generate the preview. If it shows quickly, that confirms that the feed has not been associated with that message yet.