I checked your booking object and it is still possible to book on the current date with the established time slots: https://prnt.sc/26qh6zq
Maybe the website’s server time and the customer’s local time are different, so the nearest time frame cannot be selected
I just recreated the booking category so you can choose the Stop booking time .. hours before the start (optional), now you can add this time for each booking object to be able to book the tour right away (minimum 1 hourr): https://prnt.sc/26tewhi