I own a rental house (& have a w2 job) so payed quarterly estimated taxes last year (as calculated by 2023turbotax). Now it's saying I owe a2024 underpayment penalty-why

Most people pay just over 100 percent of their prior-year income tax liability, as long as their income doesn't change dramatically. But even if you pay 100 percent of your prior year's tax, if your income has increased substantially, you may discover that you still owe more money to the IRS when you prepare your income tax return.  See this TurboTax Help.

Have you entered the four estimated tax payments into the software?

In TurboTax Online, estimated tax payments are reported as follows:

1. Down the left side of the screen, click on Federal.
2. Down the left side of the screen, click on Deductions and credits.
3. Click the down arrow to the right of Estimates and Other Taxes Paid.
4. Click to the right of Estimated Tax Payments.