Taxes owed on a 2018 tax return were due and payable on April 15, 2019. Filing an extension only extends the time to file an actual tax return, October 15, 2019, not an extension for payment of taxes.
Penalties and interest are accruing for the unpaid taxes so you should pay and file your return as soon as possible. Once the IRS has received the tax return and payment for the taxes owed, they will send you a bill in the mail for the penalties and interest owed.
