The response content cannot be parsed because the Internet Explorer engine is not available, or

In your invoke web request just use the parameter -UseBasicParsing

e.g. in your script (line 2) you should use:

$rss = Invoke-WebRequest -Uri $url -UseBasicParsing

According to the documentation, this parameter is necessary on systems where IE isn't installed or configured:

Uses the response object for HTML content without Document Object Model (DOM) parsing. This parameter is required when Internet Explorer is not installed on the computers, such as on a Server Core installation of a Windows Server operating system.

To make it work without modifying your scripts:

I found a solution here: http://wahlnetwork.com/2015/11/17/solving-the-first-launch-configuration-error-with-powershells-invoke-webrequest-cmdlet/

The error is probably coming up because IE has not yet been launched for the first time, bringing up the window below. Launch it and get through that screen, and then the error message will not come up any more. No need to modify any scripts.

You can disable need to run Internet Explorer's first launch configuration by running this PowerShell script, it will adjust corresponding registry property:

Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Internet Explorer\Main" -Name "DisableFirstRunCustomize" -Value 2

After this, WebClient will work without problems