The reason RigidBody.Velocity is not working in the newer versions of Unity is that it has been renamed to Rigidbody.linearVelocity
The same can be found in the release notes https://unity.com/releases/editor/whats-new/6000.0.23
Editor: Changed: Renamed Rigidbody.velocity to Rigidbody.linearVelocity in order for the API to be symmetric with Rigidbody.angularVelocity