Union dues are job-related expenses. Only the amount of job-related expenses that are OVER 2% of your adjusted gross income get added to your list of itemized deductions, which still must exceed your standard deduction in order to make a difference on your return. This is kind of a hard deduction to get. If you need to enter these, go to Federal>Deductions and Credits>Employment Expenses>Job-Related Expenses.
That being said, you are posting from the Free version of TurboTax which does not support itemized deductions. In order to itemize your deductions, you need an upgraded version of TT such as Deluxe (or higher). If you do not have enough deductions to itemize, then your union dues will not affect your refund or tax due. You will just receive the standard deduction.