Theresa....I would use a deodorizer but do you think there maybe a leak...can you remove the floor of the cabinet and see or is it use a sink with pipes..it could be coming from pipes.
Here are some things that worked for us: https://www.frugalfamilytimes.com/2019/04/how-to-get-rid-of-musty-smell.html?m=1
Here's an article with some ways to help your situation. Wishing you the best.
Check first to see if there is a water leak causing the odor. If not, try these items, I no particular order, to try to remove the mustiness...cat litter, baking soda, charcoal, etc. Keep using an item, changing it every day for a week until you find what is working best for you. If there are large gaps under there, fill them with expandable foam.
get some damp rid. after you find the source.