Hi Ken,
I think if you look at the documentation here https://shopify.dev/docs/admin-api/rest/reference/orders/order#cancel-2020-07
you can see that you don’t need the {order: } wrapper in your json. If you send just the json:-
{“amount”:“35.26”,“restock”:true,“reason”:“other”,“email”:true}
then it should work as you are expecting I think.
Angus