Why does the last line in cmd powershell -command error

You're starting a new Powershell session with each command. And so the variable $WebClient doesn't exist in the powershell session created in the last command.

Instead of calling powershell -Command on each line, call powershell once and run all those statements in a single session. eg:

C:\Users\david>powershell
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PS C:\Users\david> $Url = 'http://shared4.info/psequotes/files/2021/stockQuotes_$CurrentDate.csv'
PS C:\Users\david> $Path = 'C:\Users\kevin\Desktop\stockQuotes_$CurrentDate.csv'
PS C:\Users\david> $WebClient = New-Object System.Net.WebClient
PS C:\Users\david> $WebClient.DownloadFile($url, $path)

Or from a batch file like this:

powershell -Command ^
$Url = 'http://shared4.info/psequotes/files/2021/stockQuotes_$CurrentDate.csv'; ^
$Path = 'C:\Users\kevin\Desktop\stockQuotes_$CurrentDate.csv'; ^
$WebClient = New-Object System.Net.WebClient; ^
$WebClient.DownloadFile($url, $path);