Control nodes do not have a
position property. If you look at the docs:
or at the Control node's Inspector, you'll see that
rect_position is the property.
A few other comments:
rect_position is a Vector2 (as is
position), so there's no need to set the components separately.
CityName.rect_position = MousePos
works just fine.
2) You've created a new TextEdit node, but it will not be visible/usable until you add it to the tree with