If the distribution did not take place until 2019, you'll receive a 2019 Form 1099-R reporting this distribution which will be reportable on your 2019 tax return.
If this distribution was required to have been made by the end of 2018, your 2018 tax return must include Form 5329 requesting waiver of the 50% excess accumulation penalty on the amount received late. TurboTax makes this nearly impossible if there is no other reason for TurboTax to ask if you received all of the RMD for 2018, so it might be simplest to prepare Form 5329 separately and include it with your mailed tax return.
https://www.irs.gov/pub/irs-pdf/f5329.pdf
See Waiver of tax on page 8 of the instructions for Form 5329:
https://www.irs.gov/pub/irs-pdf/i5329.pdf
