If part of your Social Security benefits were taxable on your federal return, they were already included on line 2a of the Maine Schedule PTFC/STFC. Line 2b shows the benefits that were not included in the amount on line 2a.
You can check your federal tax return to see if you had any taxable Social Security benefits. Check line 20b if you filed a Form 1040 and check line 14b and if you filed a Form 1040A. (Click the screenshots below to enlarge them for reference.)