How do I enter the following information in box 12 of the W2: D 20 840.99????

That doesn't look like a valid entry for box 12 on a W-2. A box 12 entry has only two parts - a code, which is one or two letters, and an amount.

Code D is contributions to a 401(k). How much did you contribute to your 401(k) - $840.99 or $20,840.99? You should be able to find your total contributions for the year on your final pay stub for the year, or in a statement or records from the 401(k) account. If you contributed $20,840.99, then your employer just formatted the amount oddly. enter it without the space. If you contributed $840.99, just enter that amount. You will have to ask your employer what the "20" means.