I did not have a return of contribution of an IRA before April 15, 2019, but TurboTax wants me to explain details of something that didn't happen. How do I fix this?
You mistakenly entered $0 in box 2a of the Form 1099-R (or left it blank). A Form 1099-R reporting a regular distribution from a traditional IRA (code 1, 2 or 7, IRA/SEP/SIMPLE box marked) must always have box 2a equal to box 1 and have box 2b Taxable amount not determined marked.
(If the distribution was not from an IRA, you mistakenly marked the IRA/SEP/SIMPLE box despite it not being marked on the Form 1099-R provided by the payer.)