Win 11 will work with kACARS_Free - but.... the user needs to install .NET 3.5 - which doesn't come with Win11. A quick Google search for .NET 3.5 for Windows 11 will lead you to exactly how to do that.
.NET 4.8 (I think) comes with Windows 11. If adding .NET 3.5 doesn't solve the users issue, have them reinstall .NET 4 also.
Let us know if that works.