LINQPad6.exe is framework-dependent, so you must install .NET Core 3.0 or .NET Core 3.1.

The *netcore runtime* alone isn't enough: you also need NETCORE DESKTOP (or the SDK) -- X64 --

You can check that it's installed by running "Download .NET Core 3.exe".

You should end up with the following folders:
C:\Program Files\dotnet\shared\Microsoft.NETCore.App
C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App

NON-STANDARD NETCORE INSTALLATIONS
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You can install or xcopy .NET Core to a non-standard location, such as c:\users\joe\dotnet.
This is handy if you don't have admin rights to your PC.

If you do this, be sure to set the DOTNET_ROOT environment variable to the folder containing dotnet.exe.
For example:

set DOTNET_ROOT c:\users\joe\dotnet