Deductions & credits

I was not required to complete Schedule C in prior years and was not directed by the TurboTax step-by-step system to do so; I think my confusion was justifiable. Now, however, after manually completing Schedule C, TurboTax has calculated a penalty for late payment of my Social Security contribution - any ideas on why that might be? Today is April 13th so I do not think the payment can be considered as late....?