Yes, of course your can prepare and file your 2018 return using TurboTax. E-filing is open for 2018 returns until October 15, 2019. After that a 2018 return must be printed, signed and mailed.
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
