There is a very strong sewer gas smell under a bathroom sink inside the cabinet. There are no obvious drain line breaks or leaks. Any idea where this is coming from or how ?fix it

Does this smell change when you run the water at all and how well does the sink drain?
For any sewer gas to escape there has to be an opening somewhere in the waste vent or drain line possibly a small leak in the wall that didn't show up yet

Check trap connection from sink and for any tiny leaks.