Please make sure you are logged in to your Google account, Apple ID or Amazon account. If that is the case and the problem persists, it is recommendable to uninstall the app and reinstall the newest version.
Another cause could be you internet connection. If possible, check if you still have problems with a different connection. For a booking a constant connection to the internet is required.
If all of the above instructions did not help, please send us an email with a screenshot to firstname.lastname@example.org. We will then help you as fast as possible.