Last year's tax liability cannot be less than zero. If you used TurboTax for your 2020 tax return, this would have carried over on the Carryover Worksheet from line 6 of your 2020 Carryover Worksheet.
It seems likely that you made an error by manually entering the negative value as your last year's tax liability, replacing the value carried over from your 2020 TurboTax tax file. This entry is either 100% or 110%, depending on your AGI, of last year's tax liability. Revisit Other Tax Situations -> Additional Tax Payments -> Underpayment penalties and correct the entry on the 2020 Tax Liability page by entering the 2020 value from line 6 of the 2020 Carryover Worksheet.
