The distribution of your Roth IRA should be reported by your broker on a 1099-R. Typically, on the 1099-R very little, if any, of the Roth distribution is taxable. If you haven't received your 1099-R yet, contact your IRA custodian. If you have received it, check you TurboTax entries.
If Box 2a of your 1099-R is empty, you should leave it empty, and then answer the rest of the questions. The program will calculate the taxable amount.
It's hard to tell what the issue is without knowing more about your return.
You can withdraw contributions you’ve made to your Roth penalty-free at any time, for any reason (this is because you’ve already paid taxes on the contributions). Once you withdraw your contributions, you can take out up to $10,000 of your earnings for a first-time home purchase – without paying the 10% penalty. As an added bonus, if you’ve had the Roth IRA for at least five years, the withdrawn earnings are tax-free; if it’s less than five years old, the earnings are taxable.
