EXEC master.dbo.xp_cmdshell 'The user name or password is incorrect'
You should check if you have permissions or not 

exec xp_cmdshell 'net use \\SHDVNFDBV1\shared-backup\  /USER:domain\username Passw0rd1'


Do not copy to root directory as @AaronBertrand mentioned.

As a side note, why are you using SQL Server to do filesystem tasks? This should be done by PowerShell e.g. Copy-Item c:\scripts\test.txt c:\test