Here is the link to the official Microsoft .Net 9 Runtime for x64 (Windows) needed for SolmanSolitaire version 6 .1 and above 

Download .NET 9.0 Desktop Runtime (v9.0.19) - Windows x64 Installer (microsoft.com)

You can also just browse "Microsoft .Net 9 Desktop Runtime" and looking for the windows x64 installer (not the Sdk version) from Microsoft.

Required for version 6.1 of SolmanSolitaire, available on Windows 11 only.