If the US Constitutions forbids anything but gold/silver for payment, why do we have paper money?

I imagine it is because the money is issued by the federal government. In my mind, each state is still making it legal tender for debts when they require state taxes, traffic fines, fees, etc to be paid in federal reserve notes. Is there a court case that discusses this?
Update: I am talking about Art I Sec 10, US Constitution: "No state shall make anything but gold and silver coin a tender in payment of debts."
Update 2: Brandon, the gold standard was abolished decades ago, at the same time it was declared illegal for US citizens to hold gold.

Ezz, I think you're on the right track but the states are still using that paper money for payment of debts.

Is there a court case?
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