I purchased a tandem 20amp breaker which I thought was going to allow me to put 2 separate 20amp's into one open slot in the panel, however when I attempt to install it into the panel box itself there is a little silver metal piece that prevents me from pushing the breaker all the way into the panel slot, is this supposed to be I have looked at the same breaker again in the store and they all have this. I have a cutler hammer break panel 200 and the breaker itself is a cutler hammer so I am sure it is the one I need it just won't push all the way in because of this metal piece, does anyone know what the issue is? Any help is appreciated.

That means that your panel is not designed to accept tandem breakers. At least not the spot you had open.

Look at the label. It should say where tandems would fit. Also look at the model number. If it is anything like 2040 or 3040 then you can install tandems in the lower breaker spots. If it is any like 4040 then that panel will not accept any tandems.

Thanks Stan, I will check it out.

It looks like I will not be able to put a tandem in my box, is it possible to ad a sub panel on my own or do I need to contact my electrical company to do this?

Your "electrical company" doesn't add panels. For that you need an electrician, or someone experienced in wiring.