If you're outside the U.S. and it's already 4/16, set your computer's clock to a US time zone that's before midnight on 4/15 and you should be able to select 4/15.
Otherwise, to pay your IRS taxes on time, select "I will pay by check" (ignore the check payment instructions that follow) and then go to the IRS webpage https://www.irs.gov/payments to pay online.
(revised instructions 4/15/19 at 7:29 PM Mountain)
