How can I enter my 2015 tax overpayment to my 2016 estimated payments when ttax failed to do it?

You'll enter the overpayment from 2015 by clicking on the Federal Taxes tab, then Deductions & Credits, then scroll down to Estimates & Other Taxes Paid and select Other Income Taxes.

You'll then see various category headings for payments made, one of which is 2015 Refund Applied to 2016.  The first item in this section is "2015 refund applied to 2016 federal taxes."  This is where you'll enter the 2015 overpayment carried forward to 2016.

I am about to efile my 2021 returns, and have over payment of taxes for federal and state that I want to carry forward to my next year's tax returns?  Where do i do the input?

Here's the steps to apply your 2021 refund to your 2022 taxes.

@dvolkerding