It's not a bug. The key phrase in the sentence that you quoted is "will be calculated here." TurboTax calculates the amount on Form 8889 line 7. You cannot enter it yourself. TurboTax calculates it based on your age as of the end of 2023 and your answers to the questions in the HSA contributions section of the TurboTax interview.
Were you married as of the end of 2023? You can only have an additional contribution amount if you were married.
Were you enrolled in Medicare in 2023? If so, you cannot have an additional contribution amount.
For additional details, see the instructions for line 7 on page 7 of the IRS Instructions for Form 8889.
If you still think there should be an amount on line 7, go back through the HSA contributions section of the interview again and check your answers.
