When entering the test 1099-R, enter the same amount in boxes 1 and 2a, unless you know that some of your 401k money is "after tax". In that case, leave box 2a blank and check the box "Taxable amount not determined". Follow the interview carefully.
For more on this subject, see:
https://www.investopedia.com/articles/retirement/08/convert-401k-roth.asp
