Now that your Reserve with Google account has been set up, your linked Cloud Project from the previous steps has been granted access to the Maps Booking APIs. Now is a good time to complete the basic setup of those APIs, before any development work begins. You need to enable them in your Cloud Project. In the API Console, click Enable APIs > Google Maps Booking API > Enable.
Then repeat this for the Google Maps Booking API (dev) version: Enable APIs > Google Maps Booking API (dev) > Enable. The API marked with (dev) can be used to access the sandbox environment.
If you are having issues with enabling the API, please take a look at this