That’s not actually true, there are things the federal government can do. First it’s a grey area legally. The constitution says trade deals (and trade outside the borders of any one state) is the domain of the federal government.

The argument in this case would be “Is this a trade deal?”. It certainly sounds like a deal, and it involves trade, but the key technicality would be if California is giving anything in return. Are they promising anything in exchange for no or reduced tariffs or are they just asking with the promise of nothing in return? If they’re not promising anything there’s a pretty good chance they could win the argument that this isn’t a trade deal and therefore the federal government has no legal basis to intervene (although it’s worth pointing out that the current administration hasn’t particularly let legality influence their actions).

On the other hand if California is promising something in return there’s a decent chance the federal government could successfully argue that that meets the definition of a trade deal and is therefore prohibited. This also raises the question of why another country would agree to remove tariffs from California if they aren’t promising anything in return. The only answer I can come up with is to figuratively (and maybe literally at the same time) give the middle finger to Trump.