Code P on a 2018 Form 1099-R means taxable in 2017, so this income must be reported on your 2017 tax return, not your 2018 tax return. If you already filed your 2017 tax return and did not already include the taxable amount shown on this Form 1099-R, you'll need to amend yoru 2017 tax return.
2017 TurboTax's drop-down text assumes that you are entering a 2017 Form 1099-R, but in the follow-up you'll indicate to 2017 TurboTax that this is a 2018 Form 1099-R.