可能的原因是令牌不包含所需要的授权信息，或者在 API 控制器中没有正确配置身份验证和授权。首先，确认要访问的 API 是否需要身份验证和授权，然后使用适当的方法生成令牌并将其添加到请求头中。以下是一个简单的示例代码：

1. 在 Startup.cs 中配置身份验证和授权

public void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
        .AddJwtBearer(options =>
        {
            options.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = Configuration["Jwt:Issuer"],
                ValidAudience = Configuration["Jwt:Audience"],
                IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["Jwt:Key"]))
            };
        });

    services.AddAuthorization(options =>
    {
        options.AddPolicy("AuthorizedUsersOnly", policy => policy.RequireRole("Administrator", "Operator"));
    });

    services.AddControllers();
}

2. 在控制器中添加授权属性

[Authorize(Roles = "Administrator")]
[Route("api/[controller]")]
[ApiController]
public class UsersController : ControllerBase
{
    // API methods
}

3. 发送请求时添加令牌

HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
HttpResponseMessage response = await client.GetAsync("http://localhost:5000/api/users");

通过这些步骤，应该可以解决 ASP.NET Web API 授权被拒绝的问题。