我也遇到了同样的问题。如果你还没有得到答案，那么这个可能会有帮助。
DetectSentiment不适用于.NET Core。此操作只能以异步形式使用，您可以将DetectSentimentAsync用于相同目的。以下是我尝试的代码，它工作正常。但请注意，我在aws lambda函数中使用了它，您可以在您的函数中尝试相同的操作。
公共异步任务函数处理程序（LexEvent lexEvent，ILambdaContext上下文）{

String text = "It is raining today in Seattle";
        AmazonComprehendClient comprehendClient = new AmazonComprehendClient(Amazon.RegionEndpoint.USEast1);
        Console.WriteLine("Calling DetectSentiment");
        DetectSentimentRequest detectSentimentRequest = new DetectSentimentRequest()
        {
            Text = text,
            LanguageCode = "en"
        };

        DetectSentimentResponse detectSentimentResponse = await 
       comprehendClient.DetectSentimentAsync(detectSentimentRequest);
       Console.WriteLine(detectSentimentResponse.Sentiment);
        Console.WriteLine("Done");

它现在可以在.NET Core 6.0 AWS Text Sentiment中运行

using System;
using System.Threading.Tasks;
using Amazon.Comprehend;
using Amazon.Comprehend.Model;

/// <summary>
/// This example shows how to detect the overall sentiment of the supplied
/// text using the Amazon Comprehend service. The example was writing using
/// the AWS SDK for .NET version 3.7 and .NET Core 5.0.
/// </summary>
public static class DetectSentiment
{
    /// <summary>
    /// This method calls the DetetectSentimentAsync method to analyze the
    /// supplied text and determine the overal sentiment.
    /// </summary>
    public static async Task Main()
    {
        string text = "It is raining today in Seattle";

        var comprehendClient = new AmazonComprehendClient(Amazon.RegionEndpoint.USWest2);

        // Call DetectKeyPhrases API
        Console.WriteLine("Calling DetectSentiment");
        var detectSentimentRequest = new DetectSentimentRequest()
        {
            Text = text,
            LanguageCode = "en",
        };
        var detectSentimentResponse = await comprehendClient.DetectSentimentAsync(detectSentimentRequest);
        Console.WriteLine($"Sentiment: {detectSentimentResponse.Sentiment}");
        Console.WriteLine("Done");
    }
}