If you would like to add a free class (0 credits required to book) to your calendar that your clients can book, without requiring a class credit, you can do this very easily in StudioBookings.
Here is how to do that -
When adding a class to the calendar, change the default 'Credit per class ' to be 0 instead of 1. See below a screenshot of where to do this. Changing it to 0 will tell the system to not require a credit to book the class.