Hello @Jo249,
Some settings can be edited only once the listing is published. I'm not sure if the cancelation policy setting is one of them.
For my already published the setting is available under "booking settings" when I click on my listing photo.
I can then click on the "Edit" button in the "Policies" section at the end of the page.
If that's not available for your as yet unpublished listing you could publish your listing, immediately temporarily unlist it, and then set the cancelation policy and other settings. You can then list it again when you're happy with all the settings.
How do I snooze or deactivate my listing?
Steve.