Can I add a 1099-R that I failed to include in my 2020 taxes

Once you've e-filed your return, no changes can be made.  You will first need to wait for the IRS to either accept or reject your return.  IF, by chance, your return is rejected, you can add the 1099-R before re-submitting your return.  If the IRS accepts your return, you'll need to amend the return to make the correction.  You won't be able to do that until likely the end of February.

Here's a TurboTax FAQ about amending your 2020 return.  

How do I amend my TurboTax Online? 

Yes, thank you for your prompt reply. 

@pop257 My pleasure!