A state cannot secede* under the laws of the USA.

Similarly, under the laws of the Kingdom of Great Britain, the 13 colonies also couldn't secede: https://en.wikipedia.org/wiki/Proclamation_of_Rebellion

Famously, the laws didn't stop those colonies.

* T&C apply

If the United States becomes an authoritarian dictatorship then the United States as we know it ceases to exist and all bets are off.

If the Supreme Court wants to have its rulings respected they'll need to ensure that they have buy-in from all parties. Secession would be very messy. But there are borders that some states will not cross and others will, when - and if, and let's hope it does not come to pass - that happens there are lots of things on the table that the Supreme Court can no longer put a stop to. This is why Trump is as dangerous as he is: he doesn't care one bit about how the USA became the superpower that it is, he'll burn it all down for ego and personal gain, no matter what the consequences.