I need Help Filling out Box 12 i have a D 21 on my W-2 and cant figure out which one to pick.

It sounds like Code D and the amount is $21.  Code D shows the part of your pay that went toward a 401K.  This is informational only.  It shows the amount of income that is deferred and is not included in Box 1 of your W2.  Therefore it will not be taxable now but when you withdraw from the 401K in the future.  

@big_daddy94fu 

....If there is an actual $ amount alongside?

THEN the D21, is just a "D".....

..the 21 just indicates that some of the $$ contributed was to pay for some 2021 TSP/401k contributions that were not done during 2021.  The "21" isn't entered.