If Windows requests you to Install the .NET Core click on Yes.
The installation page of .NET should pop up in your browser.
(https://dotnet.microsoft.com/download/dotnet/5.0)
Click on Download x64, and open the .NET installer that you just downloaded.
After opening the installer click on the Install-Button.
When the installation is finished, close the .NET Installer.
Now you should be able to open the PolarLauncher.