How to add a stock company to Quicken 2019 on the desktop

More details, please.
Do you use Quicken Mac or Quicken for Windows? US or Canadian version?
What version, edition level (Starter, Deluxe, etc.) and release of Quicken are you using?  Click Help / About Quicken (Mac: Quicken / About Quicken) to get this information.
What version of Windows or Mac OS are you using?

The Quicken Starter Edition does not support investment accounts.

Can you please capture one or more images of the parts of your Quicken window showing the issue, sensitive information blacked out as necessary to protect your privacy but annotated to describe the situation, and attach the image(s) here?
https://community.quicken.com/discussion/7867159/faq-how-do-i-post-a-screenshot-in-the-community-from-windows#latest

It is likely you already have a security by that name or that ticker in your data file.  It may be a hidden security.  

Ctrl-Y to get to the security list.
Check box at bottom to show Hidden securities.
Click on Security header to sort by security.  Review list.
Click on Symbol to sort list by ticker symbol.  Review list.
If necessary, uncheck the "Hide" box for the applicable security.  

Hi @""Thomas Corbett" ,

In order to help, we need to know what you are running and we can tell you exactly what to do or you can call Quicken Tech Support again