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If you can't check the box to indicate you were a Non-Resident on the Pennsylvania Information Worksheet, there may be something in your return indicating you were a resident or part-year resident.   

In the PA interview, the first screen should confirm your residency (screenshot).

Check your Personal Info entries to be sure you didn't indicate that you lived in PA.   You should only indicate that you earned income in PA.

Here's more info on How to File a Non-Resident State Return.