I have created an API key in my Sparkpost panel with all permissions enabled. I now try to send a test email with the body as specified in the Sparkpost API documentation (see below). When I make this API call, I get back a 404 with this body: {"errors": [ {"message": "Resource could not be found"} ] }. I am now wondering what I am doing wrong. Which resource cannot be found? And how can I fix this?

Details of request

URL: https://api.eu.sparkpost.com/api/v1/transmissions/
HEADERS: Authorization: <MyAPIKey>
BODY:
{
  "options": {
    "open_tracking": true,
    "click_tracking": true
  },
  "metadata": {
    "user_type": "students",
    "education_level": "college"
  },
  "substitution_data": {
    "discount": "25"
  },
  "recipients": [
    {
      "address": {
        "email": "[email protected]",
        "name": "Wilma Flintstone"
      },
      "substitution_data": {
        "customer_type": "Platinum"
      }
    }
  ],
  "content": {
    "from": {
      "name": "Fred Flintstone",
      "email": "[email protected]"
    },
    "subject": "Big Christmas savings!",
    "reply_to": "Christmas Sales <[email protected]>",
    "headers": {
      "X-Customer-Campaign-ID": "christmas_campaign"
    },
    "html": "<p>Hi {{address.name}} nSave big this Christmas in your area {{place}}! nClick http://www.example.com and get a {{discount}}% discountn</p><p>Hurry, this offer is only to {{user_type}}n</p>"
  }
}