You need to add to csproj an additional switch:
<UseWindowsForms>true</UseWindowsForms>
Add it below UseWpf. Then try rebuild. After this, you should be able to use Forms namespace.
You need to add to csproj an additional switch:
<UseWindowsForms>true</UseWindowsForms>
Add it below UseWpf. Then try rebuild. After this, you should be able to use Forms namespace.