I already paid my due taxes and it still is asking me to set up a payment method so I can proceed. What is going on?

Who is "it" that is asking you to set up a payment plan?

If you are seeing this on the TurboTax website or have received an email from TurboTax to complete a payment plan, then disregard if you have paid the taxes owed.

Do you have proof of paying the taxes owed, either from a cancelled check, bank or credit card statement showing the taxes paid?

TurboTax has no way of knowing that you paid the tax. All it sees is the amount due on your tax return. If you already paid, and TurboTax is asking how you want to pay, say that you will mail a check. Then you will be able to proceed to file your tax return. TurboTax doesn't know whether you actually mail a check or pay another way, and it doesn't matter how you pay. By saying you will mail a check you are just telling TurboTax that you will take care of paying the tax due.

if you made an extension payment not using Turbotax, it does not know that the balance was paid. you may need to enter any extension payment in the tax payments worksheet