What does the code D 18 mean for a military or government W2?

Is that in box 12 on a W-2?

Code D in box 12 reports your contribution to a 401(k) plan.   You do not enter the 18, that just means that it is for the year 2018. Just enter the code D and the amount.

But, It’s not a 401K. It’s a military pension.

"18" means that the elective deferral was for 2018.  The "18" is redundant because code D by itself in box 12 of a 2018 Form W-2 means an elective deferral for 2018.