Any cancellations made by a client must be done before certain dates. The cancellation charge, if any will depend upon the date the company receives the request from client. If the booking is done via some 3rd party platform like Viator, then terms and conditions mentioned there will be applicable.
General charges applicable in the event of a cancellation shall be as follows from the day of closing booking:

Cancellation charges 11 days or more before last booking date : 10%
10-4 days: 25%
Any cancellation after that isn’t eligible for refund.
Cancellation charges for multi-day tours are mentioned while booking the tours and can be different from the details mentioned here. These details will be provided by the company before tours are booked in such a case.