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 

You have until October 15, 2019 to e-file your 2018 tax return.  If you did not make a payment of estimated taxes when you requested your extension, then you will owe the tax due when you file the return.  If you are getting a refund there is no penalty for filing after the April 15 deadline.  If you owe, however, the IRS will calculate and bill you for the interest.  TurboTax will not calculate it for you.

The IRS won't determine and bill for any penalty, interest or other balance due until you file your tax return.  The extension only serves to eliminate a late filing penalty for filing after April 15 (but by October 15) and to provide a means of making what amounts to an estimated tax payment (although reported separately on Schedule 5 from other estimated tax payments).