Overview

Reservation feature is not loading in the preview.

Solution

The slide-out Previewer uses a PWA version of the app and as such can only display features that are available to a PWA. Any features not available for PWA (refer Feature List For IOS, Android And PWA for details) will not be visible/available from there.
You can use the following alternatives to see the features that are not available to a PWA.
Preview Your App Using the Native Android Preview App
Preview Your App Using PreviewYourApp.com
Preview Your App Using the Native iOS Preview App