Urine stains are tough. The ammoonia in the urine reacts ith the tannins in the wood to cause the dark stains. Some woodworkers actully use ammonia to "fume" woods like oak to caue the wood to turn a dark color.
I have one option which is low-cost and low-effort, but has the best chance of success.
It's available at most paint stores. Mix the acid (it comes as a powder) as directed and apply to the stain. Afterwards, neutralize the acid with a wash of 1T of baking soda mixed in a pint of water. Then rinse the area with fresh water.
You may have to repeat the process two or three times.