Use either
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserRoleView}},
Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"
Or add this to your UserControl:
<UserControl
Name="myControl"
...
Then instead of using RelativeSource use binding like this:
Header={Binding Path=LeftColumnHeader, ElementName=myControl}
But actually I'm not quite sure that you still will be able to bind it that way you do it, as columns headers have some weird rules when it comes to binding. Check it: