Deductions & credits

A Business Mileage deduction may not increase your Federal Refund.

It will reduce your Business Income on Schedule C, which in turn may reduce your Taxable Income, which in turn may increase your refund.

Look at your Schedule C, Line 31.  This amount carries over to Schedule 1, Line 3, and then to Form 1040, Line 8 where you can see how much of an income deduction you received.  Unless your income is reduced by a sufficient amount, a small reduction won't change your tax amount.