Can I cancel, and what do I get back?

Yes, from your booking page, and you are told the exact amount before you confirm. Cancel inside the tour's free window and everything you paid comes back; after it, what returns depends on the policy the guide chose and on how you paid.

Every tour shows its cancellation policy before you book and again on your booking. If the guide cancels, you are refunded in full, whatever the timing.

If you paid the whole price by card, a cancellation after the free window returns the share the policy names, to the card you paid with.

If you paid a deposit, a cancellation after the free window returns nothing: only the deposit was charged, and it is kept. The rest was never charged, because it was to be paid to the guide on the day.