A 2018 1099-R with a code "P" in box 7 is taxable in 2017, the year the contribution was made, not 2018.
You must amend 2017 to report it. The 2017 1099-R interview will say that code P means "Return of contribution taxable in 2016", but the interview will ask if this it a 2017 1099-R or a 2018 1099-R. Say 2018 and the "taxable in 2016" becomes "taxable in 2017" (the year advances by 1 each year).
It is not reported in 2018 unless there were earnings in which case the earnings should be reported on a separate 2018 1099-R with a code 8 on box 7 to be reported in the year the earnings were returned.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
