My mileage at work was reimbursed for 20,000 and is under L on section 12. It is supposed to be non-taxable so why is it not coming off my income?

For reimbursed business miles to be tax exempt they must be accounted for.  

You seem to be under a general plan that does not require you to submit a record of your mileage to your employer to be reimbursed.

• Rather,
  • If your reimbursement is more than the federal rate, such as the standard mileage rate or a FAVR,
  • The amount up to the federal rate is reported in Box 12 of your W-2 form using Code L. 
  • While the amount in Box 12 isn’t taxable, the excess reimbursement will still be included in Box 1 of your W-2 form.

Please see How are taxable mileage reimbursements reported?  in Is Mileage Reimbursement Taxable Income?