Letter from IRS: since TT put a value in line 10 of schedule 2, they need form 5405. Why didn't TurboTax include this? How can I get it now that my taxes are filed?

You may need to amend your return to add the form. 

Please see this Turbo Tax FAQ on when to amend.
https://ttlc.intuit.com/community/amending/help/do-i-need-to-amend/00/27607

How do I add or remove Form 5405 for the first-time homebuyer credit?
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-add-or-remove-form-5405-for-the-first-time-homebuyer-credit/00/26153

I think I see the answer in this post, which is that the IRS made a mistake in requesting this.  I've sent them a response.  I'll see how they handle it.  Solved: Re: I need to add form 5405, First time Homebuyer ... (intuit.com)

Oh... I think I see the issue.  I bought my home in 2010 not 2008.  So the homebuyer credit repayment is waived as described here: Instructions for Form 5405 (Rev. November 2021) (irs.gov)  and here: Topic No. 611 Repayment of the First-Time Homebuyer Credit | Internal Revenue Service.  I think I probably need to file an amended return that doesn't include the $500 repayment.  We also repaid $500 last year.  Is it possible to get this money back?

Yes, 

you will need to file an amended 2020 tax year return for a refund. 

@jr135 

How to amend 2020 return

Thanks @KrisD15 I filed a form 843 to request a refund from the past two years.  Not sure if that'll work, but since it was for two years it seemed like that might be the right way.  If that's wrong I'd love to know.  Thanks!