I'm missing a using directive or assembly reference.

I pretty much have no idea what Unity is complaining about here:

I might be missing a using directive or assembly reference, but I don’t have any idea where I can find them. Actually I pretty much don’t understand this error at all. I was hoping someone on this forum could help me.

rigidbody is not defined.

You are basically using old code version in newer unity.

change:
rigidbody.velocity = movement;
to
GetComponent().velocity = movement;

I can see why are you having theese problems. You aren’d doing it wrong. I just looked, and SpaceShooter tutorial appears to be really outdated. But from what I saw in your code before, basicaly everywhere where is “rigidbody.velocity” in the tutorial you should have GetComponent() instead. That should fix this error.

EDIT:
btw If you have the possibility to use visual studio for coding instead of Monodevelop, you might wanna use it (for me it installed with unity (Visual Studio Community 2017)) It tells that “rigidbody.velocity” is outdated and what you should use instead. At least to me, you would need to set it up for use with unity.

Ok, thank you, I have another problem dealing with unsupported outdated code. Error = "
Non-convex MeshCollider with non-kinematic Rigidbody is no longer supported in Unity 5.
If you want to use a non-convex mesh either make the Rigidbody kinematic or remove the Rigidbody component. Scene hierarchy path “player”, Mesh asset path “Assets/Models/vehicle_playerShip_collider.FBX” Mesh name “player_ship_collider”… ???

Just what it says. GameObject “player” has Rigidbody and MeshCollider. This MeshCollider is non-Convex and that combination is no longer supported. What you need to do is select player, find MeshCollider component in the inspector and check the “Convex” checkbox there.

Do you know of a newer version of the Space Shooter tutorial?

Not exactly…I know only about the official one, which appears to be the outdated one. But they have the upgrade guide there as well:
https://oc.unity3d.com/index.php/s/rna2inqWBBysn6l?_ga=2.166518974.1228221880.1511730985-874604971.1507644512

Although it isn’t exactly short reading(23 pages), everything should be there. (I don’t know why they didn’t updated their videos for begginers tutorial, because it would be much simpler for everyone, but this is all what they have)