I already filed my Louisiana State Tax and received my refund however I forgot to claim my new car registration taxes. I have not filed my Federal Tax, what can i do?

You may be able to claim your car registrations on your Federal return (the new Standard Deductions of $12,000/Single and $24,000 married means that fewer people still itemize their returns), but you will need to amend your Louisiana return to claim the deduction.

Here's how to Amend My Return in TurboTax.