Check these under Scheduling, Setup:

• There is at least one service, and it is shown on the booking page.
• Someone does that service: under People, the service is selected for at least one person.
• That person can be booked online and has working hours on that day.
• The day is within your rules: not sooner than your shortest notice, and not further ahead than you allow.
• They are not on time off, and their diary is not already full.
• If a calendar is connected and set to block bookings, a busy day there removes those times.