Windows 10 PE Unable to map network drive anonymously

That's just how Windows 10 connects to anonymous shares: it checks some username and password to see if it is able to log in, and if so it allows an anonymous connection.

Apparently whatever part hides this from the user didn't make it into the PE build, so just specify a dummy username and password, like /user:user pass.

If you need to provide actual credentials you can specify a user and password like this: /user:DOMAIN\user *. The star will make sure you are prompted for your password instead of having to specify it on the command line.