According to the Doc:How to: Convert between .NET Framework and Windows Runtime streams

.NET Framework streams don't support cloning, even after conversion. If you convert a .NET Framework stream to a Windows Runtime stream and call GetInputStreamAt or GetOutputStreamAt, which call CloneStream, or if you call CloneStream directly, an exception occurs.

You need to convert a System.IO.Stream to a Windows.Storage.Streams.IRandomAccessStream. You could refer to the thread: https://stackoverflow.com/a/48152204