You didn't say which IRS instructions or formulas you are using. There are 7 different methods that can be used to calculate the tax on Form 1040 or 1040-SR line 16, depending on what types of income you have and other things in your tax return. The conditions for determining which of the 7 methods to use are explained starting on page 36 of the 2025 IRS Instructions for Form 1040 and 1040-SR.
If you used the Tax Computation Worksheet on page 80 of the IRS instructions, there is probably something in your tax return that requires using one of the other methods. The most common reason that you cannot use the Tax Computation Worksheet is that your income includes qualified dividends or long-term capital gains, which are taxed at lower rates. In that case you probably have to use the Qualified Dividends and Capital Gain Tax Worksheet, or possibly the Schedule D Tax Worksheet.
If you are using the desktop TurboTax software you can see which method TurboTax used by looking at the 7 checkboxes in the Tax Smart Worksheet in forms mode. The Tax Smart Worksheet is between lines 15 and 16 in the first section of the "Form 1040 or Form 1040-SR Worksheet."
