For 2018, the IRS totally redesigned the 1040 and also eliminated the 1040A and 1040EZ. "Redesigning" the 1040 means that they removed most of the lines on the 1040 and spread them across 6 schedules (1 through 6), so actually they really didn't simplify it at all.
The line referred to here (for the payment of tax with form 4868) is now line 71 on Schedule 5 of form 1040. Enter this amount as described above and then go looking for Schedule 5. I don't know if you are using the Online or Desktop software, but for Online, look for "Preview my 1040" and in Desktop look for Forms.
