There was nothing on 1040 line 24. That is the problem, there should have been. And the rules as to why you get a penality are probably as basic as anything:
In general, you may owe the penalty for 2019 if the total of your withholding and timely estimated tax payments didn't equal at least the smaller of:
90% of your 2019 tax, or
100% of your 2018 tax. Your 2018 tax return must cover a 12-month period.
Both should have raised a red flag and create an entry on 1040-24.
