Failed to connect to .net core 3.0 MVC "Web server 'IIS Express'."

You may not have started Visual Studio as an administrator.

There are precautions here, so please try the countermeasures.
Quick Start: Creating Your First ASP.NET Core Web App Using Visual Studio

Note

If the error message "Web server 'IIS Express' failed to connect" or about SSL certificate, close Visual Studio. Then, from the right-click context menu, use the Run as Administrator option to open Visual Studio. Then run the application again.

Or if IIS Express has not been installed/launched?

Other web server software other than IIS Express is running and competing with it.
Or you're putting some restrictions on your firewall or virus checking software.

I don't know the details of the article in English, but I'll write it down because there seems to be something to do with it.
ASP.NET Core on IIS Express-Empty error starting application

It may not be necessary, but IIS Express download link
Internet Information Services (IIS) 10.0 Express

Is IIS Express checked on individual components of Visual Studio Installer?That's why.

The following is an IIS article that says it is different from IIS Express, so I will erase it.
Below is a Q&A article from another software, but it will be helpful.
Enable IIS and Required IIS Components in Windows 8/8.1
How to install and configure PHP with IIS on Windows 8
For Windows 8.1 and later
Install IIS on Windows 10/8/7/Vista

However, there are articles like this, so maybe IIS can do it instead of IIS Express.
Visual Studio for ASP.NET Core development IIS support

By the way, I tried Windows 10 and it worked.
There was a small difference in the menu in the middle of the procedure.
When you start it for the first time, there will be a confirmation dialog related to the certificate, but once it's OK, it's done.

Windows 10 Pro 64-bit 1909 ASP.NET Core 3.1
Visual Studio Community 2019 16.4.5 As shown in the screenshot above, related workloads and IIS Express are included.
IIS is not included.