const options = {method: 'DELETE', headers: {'x-api-key': '<api-key>'}};

fetch('https://api.dcycle.io/v1/custom-kpi-datasets/{dataset_id}/assignments/{assignment_id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>"
    }
  ]
}