it's strange. CP504 is a final notice. therefore the IRS is saying it sent you prior notices which you ignored.
you can try the IRS Tax Advocate Service
https://www.taxpayeradvocate.irs.gov/notices/notice-cp504/
however since you paid it, your only option might be to file a claim for refund form 843
https://www.irs.gov/pub/irs-pdf/f843.pdf
did you complete form 8880 in Turbotax.
