This is all you need:
Copy-Item C:\work\from\Web.config -Destination C:\work\to\backup
No target name required and no New-Item.
If can be shortened for quick use:
copy C:\work\from\Web.config C:\work\to\backup
Take some time to learn how the Windows file system works. There are many books and blogs on the details of how to use the file system.
\_(ツ)_/