https://cloudsearch.googleapis.com/v1:initializeCustomer throws 500 error

Question

I am new to Google Cloud Search. My application will run outside the Google ecosystem and index data to Google Cloud search. I am working through the documentation to enable third-party support (https://developers.google.com/cloud-search/docs/guides/project-setup#5_initialize_third-party_support).

But when I send a request to /v1:initializeCustomer endpoint, I get a rather unhelpful 500 error.

curl --request POST
'https://cloudsearch.googleapis.com/v1:initializeCustomer'
...

{
  "error": {
    "code": 500,
    "message": "Internal error encountered.",
    "status": "INTERNAL"
  }
}

I'm not really sure how to figure out what might be causing this error and would greatly appreciate any guidance.