'Add actors to project role' API returns 404

I have tried directly in Chrome console (see Making HTTP Requests using Chrome Developer tools - Stack Overflow) :

fetch('https://xxx.atlassian.net/rest/api/3/project/MIB/role/10006', {
  method: 'POST',
  body: JSON.stringify({
    "user": ["xxx-accountid-xxxxx"]
  }),
  headers: {
    'Content-type': 'application/json; charset=UTF-8'
  }
})
.then(res => console.log(res))

Same result: 404 response.