If you are a W-2 employee, no work mileage is deductible on your federal return. You are allowed to enter it in the program because it may be deductible on your state return (not all states, and there are other limits.)
You can deduct work-related mileage for self-employment, but as noted, you can't deduct commuting. The rules are in chapter 4 here. https://www.irs.gov/forms-pubs/about-publication-463
I believe the problem is that sometimes, Turbotax assumes you can't do math. Turbotax wants you to tell it all your work miles (including commuting), using your mileage records. Then it wants your daily commute, and it will multiply by the number of work days and subtract from the total. For example, if you say you drove 2000 miles for work, and had a 4 mile commute, Turbotax will take 4 miles x 2 ways x 240 work days = 1920 commuting miles, so 80 miles were not commuting and would be deductible.
If you have a mileage record for your work-related trips (a log book, diary, or software tracking app) and you already didn't count the commute, then you can enter the work miles you want to deduct and put zero for the commute since you already excluded it. If your total mileage does include commuting, then make sure your total work miles for the year is more than your commute.
Or maybe all your miles are for commuting and they aren't deductible.
