Where do I claim union fees on my federal tax return because I saw that they are 100% deductible?

You can no longer claim any miscellaneous itemized deductions on Schedule A, including union due/fees. This was suspended through 2025.

HOWEVER - they may be deductible on the state return, depending on your state's laws. 

Fees for what?

Your answer probably won't matter. As CatinaT1 said, job expenses are no longer deductible.   

 Even in the "old days" (before 2018) , job expenses were only a misc. itemized deduction.  You only got to deduct that portion of  your misc. itemized deductions that exceeded 2% of your AGI,  and then only if your total  itemized deductions exceeded the standard deduction (which was doubled under the 2018 tax law. .  (2% rule explained: https://ttlc.intuit.com/questions/2902781-what-is-the-2-rule ).