After you install the WP Travel Express checkout plugin you will need to enable it by going to Trips > Settings > Payment > WP Travel Express checkout. Also, you will need to enter the client id which you will get from dashboard of Express checkout.
For any confusion you can even refer plugin official documentation given below:
Also when we tried to inspect your site URL we found that you have not activated WP Travel plugin and the Express checkout plugin. So please once kindly go through the documentation and if you are confused/ has query then please let us know.