Why do I have a urine smell under vanity in bathroom
This may be a case of mold or mildew which can emit a urine like smell. 

I lived in a lower level apartment for a time that was overly humid and this urine smell would crop up in enclosed areas or behind furniture from time to time.

In this case you need to ensure you improve ventilation/airflow in the affected areas, to stop the mold problem from getting worse.

Once you have better airflow, you can deal with the scent by putting vinegar in a spray bottle and lightly misting all the affected surfaces. Let the vinegar dry, then wipe down the surfaces with a rag and warm soapy water. If the smell persists, make a solution of baking soda dissolved in water and mist all the surfaces with that. Once again let it dry before wiping up. Be sure not to get it too wet, dampness is the enemy.  You can repeat the whole process a few times if the smell is particularly strong/persistent.

in my experience this process got rid of the odor, but if you haven't fixed the overall humidity issue the problem will keep coming back.

Also, if the cabinet is made of particle board and the problem is longstanding, the scent may have permeated the entire thing and be really hard to eliminate.