There is no mileage reimbursement code on a W2.
If you don't have a Code L on your W2, you don't enter one inside of Turbo Tax. You just type in the form exactly as it appears. Code L would be there if you had reimbursed expenses that were EXCLUDED from your income. This reduces the amount of employee expenses you can deduct.
The W2 does not detail all the information on your mileage and any reimbursements which were added to your Box 1 W2 income. You need a paystub or other record from your employer. For any expenses which were not reimbursed by your employer at all, you need to account for that on your own.
You address your employee business expenses in the section below. The input of your W2 is not where you account for your mileage. Please go to:
-Federal Taxes, then Deductions and Credits
-Choose Job-Related Expenses under Employment Expenses
From there, you need records of your mileage and any other valid job-related expenses. If you had $10,000 in job expenses, and you had a code L on your W2 for $3000, your allowable job expenses would be cut down to $7000. So, you definitely do not want to create a Code L which doesn't exist.
More questions? Please ask.