I completed and e filed my 2019 Taxes. 6,400 in taxes are due from me, but TurboTax didn’t prompt me to pay. How to I pay the amount due?

You can use IRS direct pay or use the voucher that TurboTax prepares and mail in a check with the voucher.

Here is access to direct pay: https://directpay.irs.gov/directpay/payment?execution=e1s1

Did you enter your bank account for the IRS to pull it out of your account? Or did it print out a 1040V payment voucher?

How to pay a federal tax due

https://ttlc.intuit.com/community/tax-payments/help/how-can-i-pay-my-federal-taxes/00/26212

Pay by check

https://ttlc.intuit.com/community/tax-payments/help/how-do-i-pay-my-irs-tax-due-with-a-check-or-money-order/00/26403

Pay the IRS directly at

Https://www.irs.gov/payments