在ASP.NET Core 2.1中遇到DirectoryNotFoundException的问题通常是由于文件上传路径不存在引起的。以下是解决该问题的代码示例：

1. 确保文件上传路径存在：

string uploadPath = "Your Upload Path";
if (!Directory.Exists(uploadPath))
{
    Directory.CreateDirectory(uploadPath);
}

2. 使用全路径进行文件上传：

string uploadPath = "Your Upload Path";
string fileName = "Your File Name";
string fullPath = Path.Combine(uploadPath, fileName);

using (var stream = new FileStream(fullPath, FileMode.Create))
{
    // 将文件流拷贝到目标路径
    file.CopyTo(stream);
}

3. 使用绝对路径：

string uploadPath = "Your Absolute Upload Path";
string fileName = "Your File Name";
string fullPath = Path.Combine(uploadPath, fileName);

using (var stream = new FileStream(fullPath, FileMode.Create))
{
    // 将文件流拷贝到目标路径
    file.CopyTo(stream);
}

请注意，上述代码示例假定您正在使用ASP.NET Core中的控制器和IFormFile接口来处理文件上传。确保替换"Your Upload Path"和"Your File Name"为您的实际路径和文件名。

另外，请确保您的应用程序具有足够的权限来创建和访问文件上传路径。