Yes, you can deduct your union dues as an itemized deduction on Schedule A.
Click on Federal Taxes, then Deductions & Credits, then scroll down to Employment Expenses and select Job Expenses For W-2 Income. TurboTax will show you a couple of screens with various expenses listed, and you'll have to check if any of those expenses apply to you. You'll see Union Dues listed on the 2nd screen, so check "yes" there.
Continue thru the various screens and answer the questions (many of these items may not apply to you), then you'll come to a screen that says Job-Related Expenses and you'll see two columns for writing in expenses: Description and Amount. This is where you'll write in Union Dues and the amount you paid.