Recaptcha Enterprise CreateAssessment: Invalid resource field value in the request

First time implementing reCaptcha Enterprise on the backend, and I'm receiving "Invalid resource field value in the request" when I attempt to create an Assessment. I am unsure what to troubleshoot.

I'm using a slightly-modified version of the tutorial code found here: https://cloud.google.com/recaptcha-enterprise/docs/create-assessment#c

The modification I've made is I'm using JSON credentials to authenticate since we aren't running on Google Cloud. Here is my code snippet:

            string SiteKey = "x";
            string Token = "x";
            string AssessmentName = "GarrisonTestAssessment";
            string ParentProject = "project/x";
            string RecaptchaAction = "ORDER";

            var jsonCredentials = File.ReadAllText("creds.json");
            var test = new RecaptchaEnterpriseServiceClientBuilder();
            test.JsonCredentials = jsonCredentials;

            var client = test.Build();

            CreateAssessmentRequest createAssessmentRequest = new CreateAssessmentRequest()
            {
                Assessment = new Assessment()
                {
                    Event = new Event()
                    {
                        SiteKey = SiteKey,
                        Token = Token,
                        ExpectedAction = RecaptchaAction
                    },
                    Name = AssessmentName,
                },
                Parent = ParentProject
            };

            Assessment response = client.CreateAssessment(createAssessmentRequest);

JSON Credentials look like this:

{
  "type": "service_account",
  "project_id": "x",
  "private_key_id": "xx",
  "private_key": "xxx",
  "client_email": "xxxx",
  "client_id": "xxxxx",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "xxxxxx"
}

Error message looks like this:

Grpc.Core.RpcException HResult=0x80131500 Message=Status(StatusCode="InvalidArgument", Detail="Invalid resource field value in the request.", DebugException="Grpc.Core.Internal.CoreErrorDetailException: {"created":"@1622143781.065000000","description":"Error received from peer ipv4:x.x.x.x:443","file":"......\src\core\lib\surface\call.cc","file_line":1068,"grpc_message":"Invalid resource field value in the request.","grpc_status":3}")