JWT Authorization Flow returns OAU-222 "Account does not exist"

This is a code sample (JS) for my call to retrieve and access token:

      let URL;
      let baseUrl = 'https://platform.devtest.ringcentral.com';
      let endpoint = '/restapi/oauth/token';
      URL =
        baseUrl +
        endpoint +
        '?' +
        new URLSearchParams({
          grant_type: 'urn:ietf:params:oauth:grant-type:jwt-bearer',
          assertion: JWT,
        });
      console.log('URL', URL);
      let TOKENS = await fetch(URL, {
        method: 'POST',
        headers: {
          Accept: 'application/json',
          'Content-Type': 'application/x-www-form-urlencoded',
          Authorization: `Basic ${btoa(clientId + ':' + clientSecret)}`,
        },
      }).then((res) => res.json());
      console.log('TOKENS', TOKENS);

Here's what it returns:

{ "error": "invalid_grant", "errors": [ { "errorCode": "OAU-222", "message": "Account does not exist" } ], "error_description": "Account does not exist" } 

My Client ID, Secret, and JWT are all correct from the downloaded credentials.json file. What could be the issue?

More info:

I'm building this on a service worker to retrieve a Fax (attachment) from the message store. I need the fax in PDF format (as binary).

I can't install the SDK client on a service worker. It has to be done with pure Node.js (and no npm packages). It's a CF Worker.