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;

	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;

Solved my issue with resolving the ‘Rigging’ part of UnityEngine.Animations. I looked at the project my script belonged to and saw References in Visual Studio. There I saw Unity.Animations.Rigging, deleted it and added it again. Now I can reference Unity.Animations.Rigging. Hope it helps someone

Have you installed the rigging package? I bet that’s what it is. You need to go to Window>Package Manager, then search for the rigging package there and install it.