//what is D 21 in box 12 of my W2?

A code "D" in Box 12 of a W-2 indicates Elective deferrals under a section 401(k) cash or deferred arrangement plan (including a SIMPLE 401(k) arrangement).  If the amount in Box 12, with Code D is 21, then it means you contributed $21 to your company's 401(k) plan.

Code D 21 is a contribution for 2021.  Ignore the 21.  Do you also have another D?  There is a problem if you have 2 D codes.  You might have to add them together.  

Code D 21 represents a make-up contribution for 2021 to the federal TSP under the Uniformed Services Employment and Reemployment Rights Act of 1994 (USERAA).

I have a D code as well as a D 21 code. Bothe have a dollar amount reported. What do I do with the D 21 code? Add it to the first D code amount? Enter a second D code with the amount reported for the D 21 code? Ignore the D 21 code altogether and just report for the other D code?

You can add the two amounts together and make one code D entry. They are both retirement plan contributions, so they may impact a credit for savings plan contributions that you may be entitled to.