It has no effect on your score, but…
Credit union account or any other bank account for that matter are not reported to the credit bureaus. They do not appear on your credit report, and therefore has no affect on your credit score.
The above it true unless your account shows negative balance. If your account has a negative balance, you need to pay it or else it will be closed, sold to a collection agency and then it WILL get reported to the credit bureaus and will have a negative effect on your score.
Banks tend to close accounts that show no activity after something like 5 years or so.
Your account may be already closed without you knowing it. If you want to keep it open, make a deposit or withdrawl every year or so.
If you don’t plan on using it, close it via a written letter, and ask for a confirmation that the account is closed and with $0 balance. Keep that confirmation forever.