Yes, Amex is accepted for a federal tax payment.
See this TurboTax support FAQ for payment of federal taxes - https://ttlc.intuit.com/community/tax-payments/help/can-i-pay-my-irs-taxes-with-a-credit-or-debit-ca...
Go to this IRS payment website for lower fees when using a credit card to pay the federal taxes owed - https://www.irs.gov/payments/pay-your-taxes-by-debit-or-credit-card
