Though I entered on a 1099-R Worksheet the taxable amount of a normal distribution from an IRA account, the program put in a zero on Line 15b of Form 1040 for tax year 2016. I think this is wrong?

[Just-Lisa-Now-]

Does the taxpayer have a non deductible IRA basis that wipes out the taxability? 

[Canice]

No.  It was a normal distribution from a traditional IRA.  All is taxable I think, unless there is something I am missing.  The taxpayer died in 2017 and had not filed tax returns for several years.  I have done the 2012, 2013, 2014, 2015, and 2017 Form 1040s without any problem.  On those other years, just like 2016, I entered the amount of the gross distribution and the amount that is taxable (the same each year) on a 1099-R worksheet and the program filled in Form 1040 showing a number only on Line 15b, the taxable amount correctly.

Doing the same thing on the 2016 return, the program put the amount of the distribution on Form 1040, line 15a, and put a zero in on line 15b, the taxable amount.

And, to make matters worse, I cannot seem to correct it using the override feature, as it is greyed out.

[Canice]

Oh.  I found my mistake.  I entered the value of the IRA in the wrong place, saying that it was the taxpayer's basis, when, in fact, he had a zero basis.  Your comment was truly astute!!!!