Photos of the code:

If that doesn't help read this post and it's comment: The type was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built

Try adding the reference in your code with something like this

<Window x:Class="YourClassName"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="StackPanel" Height="160" Width="300">

Some links:

https://www.infragistics.com/help/wpf/adding-assembly-to-a-visual-studio-project

https://wpf-tutorial.com/panels/stackpanel/

https://docs.microsoft.com/en-us/dotnet/desktop/wpf/advanced/xaml-namespaces-and-namespace-mapping-for-wpf-xaml?view=netframeworkdesktop-4.8

Hope these can help explain a bit more