By default, income is treated as received equally throughout the year but estimated tax payments are credited for the tax quarter in which they are paid. Because your conversion is being treated by default as having been received partially in each of the four tax quarters, you have underpayments for the earlier quarters. To treat the income resulting from the Roth conversion as having been received all in Q4 your tax return must include Schedule AI of Form 2210 (which TurboTax refers to as Form 2210AI) to allocate all of your income, tax withholding, estimated tax payments and any tax credits to the particular tax quarters to which they correspond. This is a rather onerous task. In this case your Roth conversion and the estimated tax payment would both be allocated to Q4, but you'll need to go through your personal records to determine the quarters to which to allocate other amounts.
When TurboTax indicates that you have an underpayment penalty, TurboTax will ask if you want to annualize. Answer Yes to go through the process.
