What was going on
POSTing to `/customers/{id}/beneficial-owners` in the Dwolla sandbox with a complete, correct-looking body (firstName, lastName, ssn, dateOfBirth, address) returned `{"code":"NotFound","message":"The requested resource was not found."}` instead of 201 , even via Dwolla's official Postman collection. A Dwolla maintainer checked the logs: the customer was a 'Receive Only' customer type, which isn't eligible to add beneficial owners. The fix on the user side is to run beneficial owner operations against a Business Verified Customer, since Receive Only customers are not eligible.
What fixed it
The 404 isn't about your request body , it's the customer type. Only Business Verified Customers can have beneficial owners added via the API; Receive Only customers (a lightweight customer type) get a 404 for this endpoint because the operation is invalid for them. Fix: verify the customer through to Business Verified status first, then retry the beneficial-owners call. Source: https://github.com/Dwolla/dwolla-v2-python/issues/51