If you are just getting started with Booker25, there is some initial setup you must perform. Please check out Booker25 Clean Install for the details.
If you already have Booker25 installed, you can upgrade to the latest version depending on your situation:
If you have created or refreshed a sandbox based on your production environment where Booker25 is installed, the configuration will probably not have copied over correctly. This goes for all sandboxes except Full Copy sandboxes. If this applies to you, please see: Reset configuration data
If you want to migrate reservation data between orgs, or from another system to Booker25, please see the following article: Booker25 Reservation Data Import