Hey there 
Maybe some of you - as I do -, following the course in a more recent Unity version.
As of today in Unity 6000.x the “velocity” property become obsolete and changed to “linearVelocity” wich also can brake down directly to both X and Y axis.
- linearVelocity - For X and Y axis
- linearVelocityX - For X axis
- linearVelocityY - For Y axis
To follow Rick’s instructions closer, can use the new Vector2 playerVelocity variable with linearVelocity as the following:
void Run()
{
Vector2 playerVelocity = new Vector2 (moveInput.x * runSpeed, myRigidbody.linearVelocityY);
myRigidbody.linearVelocity = playerVelocity;
}
This will take in consideration both X and Y axis velocity of the RigidBody
My simple solution was to using linearVelocityX and get only the X axis value from the Vector2, without declaring a new Vector2 variable:
void Run()
{
myRigidbody.linearVelocityX = moveInput.x * runSpeed;
}
With linearVelocityX it will take only the X axis value from the Vector2