You received a letter from the IRS about the missing 1099R? Does the letter tell you the amount that you owe for that additional income? Does the letter give you some instructions for how to pay the amount you owe?
TurboTax no longer has or supports that old program. Best to have the IRS assess your added tax and interest and pay it.
You would have to amend the 2018 tax return using Form 1040-X to add the Form 1099-R.
TurboTax does not support tax year 2018.
If the amended 2018 tax return resulted in a tax refund the refund will not be paid since it after the due date of April 15, 2022 for filing a return with a tax refund.
You can seek out a local tax professional to assist you in amending the 2018 return. If the resultant 2018 amended return has taxes owed then you need to file as soon as possible since penalties and interest will continue to accrue until the taxes are paid.
I RECEIVED A LETTER FROM THE IRS/STATE
TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.
(what the letters from the IRS mean)
I GOT A LETTER FROM THE IRS
https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs
I AM BEING AUDITED
If you purchased Audit Defense:
https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited
AUDIT SUPPORT
TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support. At the page below, click on the blue button that says "Get Help from TurboTax Support". Then it will ask you what tax year is your letter. Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.
https://support.turbotax.intuit.com/irs-notice/audit-support/