Like the error message says you can’t return a value type, you can return either a string like “true”, “1”, raw byte[] or ideally a Typed Response DTO.
Because it’s trying to save you from interoperability issues from trying to use Value Types as Service Responses (i.e. they wont work in some Native Languages). It’s highly recommended (i.e known to fail) to follow StrictMode guidelines and only return reference types, i.e. wrap the boolean in a string if you need to.