Why am I still showing a tax liability related to nanny taxes on Schedule H after adding the estimated taxes under credits and deductions? I paid in full already.

Even though you have made the payments toward the Schedule H liability, the liability will still be broken out separately on your final tax return. 

Schedule H isn't a stand-alone tax form, but a part of your individual return.  The tax calculated on this schedule becomes part of your overall tax liability and the schedule tells the IRS where to direct the funds.

The payments that you make throughout the year are lumped in with all other tax payments you made (such as withholding from your paycheck) and subtracted from your total liability, including the "nanny tax".  The net result is your balance due or refund amount.