Your customer can not book online? Customers can not find a free moment?

That will usually lie with the institutions of knowledge. You can easily check these settings in the following way
1. Settings
2. Users
3. Select a user in the left column
4. Press "Knowledge"
5. Verify that the service included in the menu for which the customer tried to book is all enabled.
6. Press "Save"

Be sure to check all users of their knowledge to see if everything is actually switched on.