Why isn't box 6c checked since I received a lump sum payment from social security for 7 years back. I entered all the information.

It may be because the lump sum payment did not make a difference in your taxable social security income in a previous year. Depending on the amount of the lump sum payment and your income and social security benefits received in a previous year, it may be that adding the lump sum payment to that year did not change the taxable amount of your social security benefits in that year. In that case the lump sum election would not be in effect, so the box in line 6(c) on your Form 1040 would not be checked.