Mechanical APDL commands don't work in Rigid Dynamics because it is not using the Mechanical APDL solver.  Delete the Command Object.

What does it mean to have a torsional spring to ground?  How does that work when the body is also translating?

Instead of a torsional spring, you could have two vertical longitudinal springs from the first body to the second body that are parallel and separated in X.  Together, they have the same stiffness as the one spring when the ellipsiodal body translates but does not rotate, but when the ellispoidal body rotates, they provide a restoring torque.