have you gone thru the section in Other Tax Situations / Underpayment Penalty to make sure TT has your 2023 info and the safe harbor penalty calcs on Form 2210 are correct. You may also be able to reduce the penalty if you adopt the Annualized Income method depending on your situation.
Did you pay any estimated tax (ES) last year either regularly or specifically for this cap gain event? When you have a large unexpected income event such as Roth conversion or large cap gain you can pay one-off ES and then use Form 2210 Annualized Income method to show the timing, otherwise you will get penalized because the calcs assume by default your total income was earned evenly thru the year and your estimated taxes need to match (withholding is also assumed to be paid evenly thru the year, it's just ES which come with specific dates) and the penalty is calculated quarterly - if you took the cap gain later in the year and don't adopt the AI method it will assume a quarter of that gain was in Q1 and penalize you since then etc.
The "safe harbor" to avoid penalty is to have paid "timely" during the year via withholding or quarterly ES either 100% of 2023 tax (110% if AGI > 150k) or 90% of 2024 tax, whichever is smaller. If you didn't hit either of those you will certainly have a penalty.
The other issue you may now see is TT by default will generate ES vouchers for 2025 based on paying "100/110% of 2024 tax" in 2025 thru withholding or estimated taxes, which may now be too high if 2024 had some one-off high income. You can go through Other Tax Situations / Form W4 and Estimated Taxes to provide estimates for 2025 to see if you can then adopt "90% of 2025 tax" instead, and determine if any withholding is enough or you need to pay ES.
