You misunderstand how capital gains tax rates apply. It's not 0% on the first $40,400. Your personal capital gains tax rate is 0% if your filing status is single and your total taxable income is under $40,400. YYour entire capital gain of $45,000 is taxable at the 15% rate since your taxable income exceeds $40,400. TurboTax is calcuating your tax properly.
If you cannot pay this tax when you file you should still file your return on time and pay as much as you can (or at the very least file an extension of time to file- which does not extend time to pay).
The IRS will let you pay in installments if necessary. The penalties for filing late are greater than the penalties for paying late.
