Receiving a 404 message when creating an Envelope

Hello, I’m trying to create an envelope using a existing connection (successful) to Docusign, but when I try to create an empty envelope, I get an 404 error (File or Directory not Found)

I’m not pretty sure of which value should EnvelopeDefinition have, I’m sending a New EnvelopeDefinition object, with the right account Id, but always get the same.