I have done Roth conversions the past handful of years and have had Form 8606 produced within TurboTax. In looking back at my 2021 tax records, however, I see that 8606 is not included. This said, on my 2021 return 1040 4a = 12,000, 1040 4b = 0, and Schedule 1 line 20 is blank. Do I need to do/amend anything since Form 8606 does not appear in my 2021 tax return? FYI: The amount is 12,000 and not 6,000 because my wife and I file a joint return & both do a Roth conversion.
Form 8606 is required for any Roth conversion, so if you both did Roth conversions in 2021 your 2021 tax return is required to include two Forms 8606 (Part II), one for each of you.
If you both made nondeductible contributions for 2021, Form 8606 (Part I) is also required for those. The fact that Form 1040 line 4a has $12,000 while line 4b has $0 and Schedule 1 line 20 is blank seems to imply that you correctly reported the Roth conversions but you somehow failed to print the Forms 8606 even though they were present. Look to see if any rollover or other notation is present on line 4b that should not be there that might suggest that the Roth conversion was not correctly reported.
You should obtain a transcript of your filed tax return to see what was actually filed (although if you paper filed your tax return might not yet have been processed and the transcript would not yet be available) and see if the Forms 8606 are present: https://www.irs.gov/individuals/get-transcript