What appears to be the problem here? So Chinese online businesses are selling chemicals to Mexican customers which can be used to produce fentanyl? Then Mexicans smuggle this into America?

The USA could simply protect their border and 99% of the supply would dry up (in addition to great reductions in crime that affects ordinary Americans). But then the Democrats wouldn't have a steady flow of illegal (and soon to be legal) voters, so that cannot be allowed to happen.