You enter this 1099 on your 2019 return, you do not need to amend the 2018 return. While it was related to the 2018 tax year, the transaction itself (return of the contribution) occurred in 2019 so that is when it is reported.
The Code P tells TurboTax and the IRS that it was related to 2018, and is not taxable. It should not change your refund or balance due once you have entered in the Code P and all of the relevant data.
The "taxable amount not determined" box is typically checked for distributions of this nature as the custodian does not know about any other transactions you may have, and it simply means that the custodian is not assuming responsibility for determining the taxable amount (which they may do in certain other types of distributions reported on this same form such as certain types of pension distributions.) This is normal and also doesn't affect the taxation.
Box 2a should be blank on the form and in TurboTax since the transaction is not taxable - this is the correct way for the form to be reported to you.
Enter the form into 2019 software exactly as it appears so that it will match in the IRS database. It will not cause your tax to increase.
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