StateMachineBehaviourで コルーチンを使う。

public class SpratCount : StateMachineBehaviour
{
 //コルーチンにはmonoが必要
 private MonoBehaviour monoBehaviour = null;
 public void StateMachineBehaviour(MonoBehaviour monoBehaviour)
 {
   this.monoBehaviour = monoBehaviour;
 }

 override public void OnStateEnter(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
 {
   StateMachineBehaviour(animator.GetComponent<MonoBehaviour>());
   this.monoBehaviour.StartCoroutine(EjacuationCount(animator));
 }

 //終了までの秒数
 private IEnumerator EjacuationCount(Animator animator)
 {
   //3秒
   yield return new WaitForSeconds(3f);
   animator.SetTrigger("Finish");
 }
}

コルーチンにはmonoBehaviour必要なんすね。