Have you checked the path inside the container before running the copy command?
I don’t use windows containers usually don’t remember the proper syntax to refer to a container path but I cans ee that you sometimes used forward slashes, sometimes backslashes.
Try this
docker container create --name dummy -v myvolume:C:/root hello-world
The source: Docker / Windows Container: how to mount a host folder as data volume on Windows 2016 - Super User
but I am sure it is also somewhere in Microsoft’s documentation I just tried a very quick search.
Ps.: Please, use code blocks like I did to make sure all your commands are shown as you post them