In Gen-2, the preview button is enabled or disabled depending on the prompting mode you're currently using. If you're attempting to generate a preview while using Image to Video or Image + Text to Video, you will not be able to, as the preview button will remain greyed out. This is because the image input itself is used as the preview.
Alternatively, the preview button is very occasionally disabled for Basic Plan users under rare instances where an extremely high number of users are concurrently generating. As these occasions are extremely infrequent and immediately responded to, just check back in a couple hours for restored access.