“I don’t know” is a perfectly acceptable answer. It shows that you’re not going to make something up just to please a customer.
“I don’t know, let me find out” is an even better response. It demonstrates your willingness to go the extra mile and solve a problem for someone.
When we work in an area where it’s easy to bend the scope of what we know to make ourselves out to be more competent than we are, we’re doing a disservice to those we serve.
It’s ok not to know something. Go find it out.