You can set that some classes/courses are available only for customers who purchased a prepaid package.
1. Go to Settings>Classes, Courses, Events
2. Click on the specific class/course
3. Go to the General tab
4. In the Package only section, select Customers can make a booking only after purchasing a prepaid package.
How can customers book classes that require the purchase of prepaid credits?
When customers visit your booking page and click on that class, they will be asked to sign in the customer area and buy/use their prepaid package