The IRS will accept cash.[1] For small payments, they have an outsourced setup with Dollar General, Family Dollar, CVS Pharmacy, Walgreens, Pilot Travel Centers, 7-Eleven, Speedway, Kum & Go, Royal Farms, Go Mart, and Kwik Trip to accept payments up to US$500. There's a US$1.50 fee. So people who have no bank account and not to pay can pay easily. For bigger payments, cash payments are accepted at an IRS Taxpayer Assistance Center, after making an appointment 30 to 60 days in advance.

[1] https://www.irs.gov/payments/pay-your-taxes-with-cash