An object reference is required to access non-static member `UnityEngine.Rigidbody2D.isKinematic'

using UnityEngine;
using System.Collections;

public class BlackBirdDrag : MonoBehaviour {

	public float maxStreatch = 3.0f;
	public LineRenderer catapultLineFront;
	public LineRenderer catapultLineBack;

	private SpringJoint2D spring;
	private Transform catapult;
	private Ray rayToMouse;
	private Ray leftCatapultToProjectile;
	private float maxStretchSqr;
	private bool clickedOn;
	private float circlRadius;
	private Vector2 prevVelocity;
	//private GameObject circle;



	void Awake () {
		spring = GetComponent <SpringJoint2D> ();
		catapult = spring.connectedBody.transform;
	}
	
	void Start () {

		LineRendererSetup ();
		rayToMouse = new Ray(catapult.position, Vector3.zero);
		leftCatapultToProjectile = new Ray(catapultLineFront.transform.position, Vector3.zero);
		maxStretchSqr = maxStreatch * maxStreatch;
		CircleCollider2D circle = Collider2D as CircleCollider2D;
		circlRadius = circle.radius;
	}
	

	void Update () {
		if(clickedOn)
			Dragging ();

		if(spring != null) {
			if(!Rigidbody2D.isKinematic && prevVelocity.sqrMagnitude > Rigidbody2D.velocity.sqrMagnitude) {
				Destroy (spring);
				Rigidbody2D.velocity = prevVelocity;
			}
			if(!clickedOn)
				prevVelocity = Rigidbody2D.velocity;

			LineRendererupdate ();

		}else{
			catapultLineFront.enabled = false;
			catapultLineBack.enabled = false;
		}
	
	}

	void LineRendererSetup () {
		catapultLineBack.SetPosition(0,catapultLineBack.transform.position);
		catapultLineFront.SetPosition(0,catapultLineFront.transform.position);

		catapultLineBack.sortingLayerName = "foreground";
		catapultLineFront.sortingLayerName = "foreground";

		catapultLineBack.sortingOrder = 1;
		catapultLineFront.sortingOrder = 3;

	}

	void OnMouseDown () {
		spring.enabled = false;
		clickedOn = true;
	}

	void OnMouseUp () {
		spring.enabled = true;
		Rigidbody2D.isKinematic = false;
		clickedOn = false;
	}

	void LineRendererupdate () {
		Vector2 catapultToProjectile = transform.position - catapultLineFront.transform.position;
		leftCatapultToProjectile.direction = catapultToProjectile;
		Vector3 holdpoint = leftCatapultToProjectile.GetPoint(catapultToProjectile.magnitude + circlRadius);
		catapultLineBack.SetPosition(1, holdpoint);
		catapultLineFront.SetPosition(1, holdpoint);
	}

	void Dragging () {
		Vector3 mouseWorldPoint = Camera.main.ScreenToWorldPoint(Input.mousePosition);
		Vector2 catapultToMouse = mouseWorldPoint - catapult.position;

		if(catapultToMouse.sqrMagnitude > maxStretchSqr) {
			rayToMouse.direction = catapultToMouse;
			mouseWorldPoint = rayToMouse.GetPoint(maxStreatch);
		}

		mouseWorldPoint.z = 0f;
		transform.position = mouseWorldPoint;
	}

}

Rigidbody2D - class name

rigidbody2D - instance name

Replace capital R with lowercase r.