Based on your reply it seems that you were not eligible for the Retirement Savings Contributions Credit, but if you were, it's likely too late to amend to claim the credit for 2015. However, 2016 and later years are still open for amendment to claim a refund.
You and TurboTax will need to know your basis in Roth IRA contributions if you ever make a nonqualified distribution from your Roth IRAs. Allowing TurboTax to track your basis now makes sure that that information is available if it's ever needed. You can make the correction to your Roth IRA contribution basis in 2019 TurboTax when you prepare your 2019 tax return, no need to do pro forma amendments to 2015 through 2018 to get that information into TurboTax.
