That's a tough sell; the US makes a lot of errors. Assuming "public policy" means "domestic"; their industrial policy promoting deindustrialisation seems like a bigger error. After a few moments of thought I'd probably argue the Bush-Obama era establishment of a massive internal surveillance apparatus (including a secret court) is a strong contender for biggest unforced error in living memory. There might be bigger ones but the US citizens lost a remarkable amount there. It is no longer possible to know what the laws of the US are, because some of them are secret. And the use of spying and federal harassment of political opponents seems to have been steadily normalising.
Even just background over-regulation on starting and running new businesses is probably going to cause more damage to poor people than legalising sports gambling.