Your project is probably configured as a WinForms project, or possibly as a class library. If it’s created as either of these, you are only able to add a WPF UserControl, unfortunately.
Of course, there’s no technical reason for this limitation, so you can copy/paste one from another project or recreate/change your project to be a WPF project.
I’m not sure what you change exactly to make it a WPF project in VS’s eyes. You might try creating a new project and diffing it to your current project. You will most likely have to do some text editing on your .csproj file.