The first error is because you’ve capitalised the first ‘C’ by mistake, and are therefore referencing the static system class ConstantForce instead of the public accessor for a ConstantForce instance. Try constantForce.relativeForce instead.
Second error is because you need to “new” up a new Vector3 instance to assign.
So you’re fixed code would be: constantForce.relativeForce = new Vector3(0,-200,0);