When editing/creating your Thank you pages, you can see a preview by clicking on the ‘YITH’ icon on the upper right side of the page.
Now a new button will show (Preview as YITH Custom Thank You Page). Click on it to see the preview.
If there are no orders in your store yet, the Thank You page will show based on a ‘dummy order’. If you do have previous orders, the plugin will use a random completed order, unless you specify an existing order ID in the related block options.