If you have an extension to file your 2018 tax return, you have until October 15, 2019 to file the return. After that, e-filing closes for 2018 returns and you would have to print, sign and mail it.
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