Check of PA state return notes following error: "Rev-1630: Prior yr taxable income must be entered." What is this referring to?

REV-1630 is referring to Underpayment of Estimated Tax by Individuals. By looking at your prior taxable income, Pennsylvania can compute to see if you should owe a penalty. This number would be from your 2017 Pennsylvania return Form PA-40 Line 12.

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Here is the Pennsylvania Form:

https://www.revenue.pa.gov/FormsandPublications/FormsforIndividuals/PIT/Documents/2018/2018_rev-1630...