I can't be sure since I can't see your return, but the US tax system is a "pay-as-you-go system". That is, normally, if you will owe more than $1,000 in tax, you should have paid it by the end of the year.
Most taxpayers are employees, so they can do this through withholding, but others like the self-employed must pay estimated taxes on a quarterly basis. Please see this IRS webpage.
You can make quarterly payments online at Payments. It is also likely that if you owed a penalty for the Underpayment of Estimated Taxes, the TurboTax will generate form 1040-ES forms as a suggestion. That is, you can pay whatever seems appropriate; these numbers are not fixed in stone.
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