On Military W2 box 12 what's the 20 when box12 shows "D 20 3,212.88" on W2? W/turbo input, there's only a Code portion

Probably a typo or misplaced digits.   Code D is retirement plan deferrals and Code DD is health insurance paid for by the government.   There are no numbers in those code.   Verify with Defense Finance and Accounting Service

What do the letter codes in Box 12 of my W-2 mean?

Military Pay and Pensions

20 is for the year, 2020.   It might have been a 2019 contribution.  Ignore the 20.