Timeline Mode

There is no Preview button because you can preview your video anytime by clicking the Play button.

You can start from anywhere by moving the playhead.

Storyboard Mode

When you are done creating your video, you can click the "Preview" button on the top right corner of the page to preview it in real-time. If you want to make additional changes, you can close the preview window and go back to fine-tune it.

Besides, you can also click the Play button to preview your work when you are editing.