我认为您需要将QuestionaireAnswerRequestModel字段设置为Answers属性，例如.添加{get;放;}:

I think that you need to make QuestionaireAnswerRequestModel field Answers a property eg. add { get; set; } to it:

public class QuestionaireAnswerRequestModel
{
    public string UserName { get; set; }
    public IEnumerable<QuestionAnswerRequestModel> Answers { get; set; }
}

从MSDN:

复杂类型必须具有公共默认构造函数和公共可写属性才能绑定.发生模型绑定时，将使用公共默认构造函数实例化该类.

A complex type must have a public default constructor and public writable properties to bind. When model binding occurs, the class is instantiated using the public default constructor.

来源: https://docs.microsoft.com/zh-cn/aspnet/core/mvc/models/model-binding?view = aspnetcore-3.1