Enter 0 as your AGI if you didn't file a return last year.
If you filed a 2018 return but don't have last year's AGI (or your extension got rejected), your best bet is to pay your taxes at IRS Direct Pay. It's free and also takes care of your extension.
- Go to IRS Direct Pay and select Make a Payment.
- Select Extension as your reason for payment.
- Select Continue and follow the instructions to completion. Submitting payment this way also gives you an extension until October 15.