If your clients are seeing this screen when booking online (as opposed to time slots), it's due to one (or more) of the following reasons:
Your service has no assigned personnel.
Your service's "Show availability online" setting is disabled.
The "Show availability online" setting is disabled for all personnel assigned to the service.
Your service's "Time selection mode" is set to "Ask client's availability. Instead, set it to any other listed option.
See here to learn more about time slots and related settings.