You are six months too late to request an extension--you needed to do that no later than April 15, 2019, and you would have had to pay your estimated tax due at that time. An extension does not give you extra time to pay. it only gives you extra time to get your documents in order and prepare the Form 1040 to file.
So now you are just plain late to file your 2018 return, and you will owe penalties and interest. File your return as soon as possible and pay as much as you can now. AFTER you have filed the return, you can request a payment plan from the IRS.
https://www.irs.gov/payments/payment-plans-installment-agreements
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. If you e-file on October 15, and your e-file is rejected you will have until October 20 to fix it and re-file it.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
