It looks like you will need to make sure the references are correct for the project. When I added mscorlib and PresentationFramework to the PowerShell Project, it started to work for me.
PresentationFramework is added automatically when I create the PS Solution and add the WPF, but I had to add the mscorelib reference and I do get intellisense, but now I get an error that Window does not support direct content.
I found a post that I needed to add a reference to system.xaml. And when I did that, I get a different error.
Apologies for the gaps in my knowledge… Just starting to play with WPF.
No worries. This shouldn’t be your problem to solve. The extension should add these references properly. Let me investigate a bit to see which other assembly reference may be missing.