I suspect that this is a 2019 Form 1099-R that includes code P in box 7 for a return of excess contribuiton. If so, TurboTax is reminding you that the amount shown in box 2a of this form was required to have been included in income on line 4b of your 2018 Form 1040. If box 7 also has code 1 or code J in box 7, the amount in box 2a also needs to be present on 2018 Form 5329, generally subject to an early-distribution penalty. If your 2018 tax return does already not include these, you must amend.
If the Form 1099-R has code R for a recharacterization instead of code P for a return of excess contribution, your 2018 tax return must reflect the end result of the recharacterization, as if you had originally made the contribution to the account where the money ended up. If it does not, you must amend your 2018 tax return.
What was wrong with your 2018 1099R? Did you get a corrected one from the plan?
How to Amend 2018
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579
It says that your 1099-R report earnings on a contribution you made in 2018. This info needs to go on my 2018 return. But I'm not sure what info from my 2019 1099-R needs to go in what spot on my 2018 1099-R.
I suspect that this is a 2019 Form 1099-R that includes code P in box 7 for a return of excess contribuiton. If so, TurboTax is reminding you that the amount shown in box 2a of this form was required to have been included in income on line 4b of your 2018 Form 1040. If box 7 also has code 1 or code J in box 7, the amount in box 2a also needs to be present on 2018 Form 5329, generally subject to an early-distribution penalty. If your 2018 tax return does already not include these, you must amend.
If the Form 1099-R has code R for a recharacterization instead of code P for a return of excess contribution, your 2018 tax return must reflect the end result of the recharacterization, as if you had originally made the contribution to the account where the money ended up. If it does not, you must amend your 2018 tax return.