You need to finish preparing your 2018 tax return. When you file it, you can choose several ways to pay the balance that is due. Enter a credit/debit account to allow the IRS to pull the payment out of your account, pay directly on the IRS website, or mail in the 1040V voucher with a check or money order--postmarked by October 15, 2019.
Your extension gave you extra time to prepare your 2018 tax return documents. It did not give you extra time to pay. If you did not pay your estimated 2018 tax due by April 15, 2019 then when the IRS receives your 2018 return and your payment, they will calculate your penalty/interest and bill you later. TurboTax will not calculate the penalty or interest.
You have until October 15, 2019 to e-file your 2018 tax return. After that, e-filing for 2018 will be shutting down, and a 2018 return will have to be printed, signed and mailed instead. The online software will also be shutting down, so if you are not finished preparing your 2018 return by then, you will have to download desktop software to a full PC or Mac to finish the return.
When you requested your extension you were supposed to pay your estimated tax due. Did you enter the payment you made into your tax return? Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes This will show up on line 17 of your Form 1040
If you have taxes owed, then in the File section of the TurboTax program Step 2 you are given several options on how to pay those taxes.
For federal taxes owed you can always use the IRS payments website Direct Pay - https://www.irs.gov/payments