If your clients see this screen when booking online, it may be due to the fact that your "Limit time before" and "Limit time in advance" settings (My Business tab > Booking rules) are in conflict.

More precisely, if your "Limit time in advance" value is less than your "Limit time before" value, no booking will be possible.

To remedy this, ensure your "Limit time in advance" setting (if enabled) value is greater than your "Limit time before" value.