c-sharp class can not reference Animation rigging package types

Despite adding the 'using UnityEngine.Animations.Rigging' header atop the script, it still cant resolve the 'Rigging' class inside 'Animations'. Specifically, it can't reference the 'TwoBoneIKConstraint' class. It's a simple class as seen below. The script can use other unity types just fine, so why not the types from the animation rigging package? Here's the script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Animations.Rigging;
 
 public class TwoBoneIkWeightByAnimationTrack
 {
     public bool Graduating { get; set; }
 
     public void GraduateIkWeight(MonoBehaviour caller, Animator animator, string animationState,
         List<TwoBoneIKConstraint> twoBoneIkConstraints, bool inverse)
     {
         if ( Graduating ) return;
         caller.StartCoroutine(GraduateWeight(
             animator,
             animationState,
             twoBoneIkConstraints,
             inverse));
     }
 
     private IEnumerator GraduateWeight(
         Animator animator,
         string animationState,
         List<TwoBoneIKConstraint> twoBoneIkConstraints,
         bool inverse)
     {
         Graduating = true;
 
         AnimatorStateInfo animStateInfo = animator.GetCurrentAnimatorStateInfo(0);
         if ( !animStateInfo.IsName(animationState) ) yield break;
 
         float percentComplete = 0f;
 
         while ( percentComplete < 1f )
         {
             foreach ( var ikConstraint in twoBoneIkConstraints )
             {
                 float normalizedTime = animStateInfo.normalizedTime;
                 percentComplete = normalizedTime - Mathf.Floor(normalizedTime);
                 ikConstraint.weight = inverse ? (1f - percentComplete) : percentComplete;
             }
         }
 
         Graduating = false;
     }
 }