Can a credit union charge a fee to process a child support bank garnishment even when they were unable to complete it do to the fact there was less than the guidelines allowed in the account.

My credit union attempted to comply with a child support order but was unable to because there wasn't at least $3500 in it and therefore they were fortunately unable to garnish the money. What I want to know is
Whether the credit union is legally able to charge the processing fee. This is the 3rd time they have attempted and the bank has never charged us before.

Sure they can. They still had to go through the process. So they had to expend effort and can ask for compensation.