Single tax rates are higher then married filing joint tax rates, and the standard deduction for a couple is twice that of a single.
You may indeed owe a penalty, but you need to go through the interview for Underpayment of estimated tax to see if you can reduce it.
Do that under the Federal taxes tab,
Other Tax Situations in the menu line below,
then Underpayment Penalties. Sometimes entering the additional information requested there can reduce your penalty.