America wasn’t legally allowed to separate from England, either, but we did it anyway.
The fact is, a people, any people, have an inherent right to separate the political bands that tie them to a larger political union, should they determine that that union has become detrimental to their well being.
But we didn't do so - not legally anyway. We openly rebelled against their authority. And we took up arms to defend that position.
As a practical matter, secession would be a difficult process full of messy details.
Suppose a state voted to secede from the union.
A vote would not be unanimous. It might be a strong majority, say 60% to 40%, but you would still have a huge number of people in the state who want to remain in the union.
Would that 40% minority be required to move out of their state, in order to retain their American citizenship?
What of matters such as that state’s share of the national debt? Would they be required to pay it off?
What about issues such as Social Security benefits paid for and earned by the residents of a state which was no longer in the union?
Would dual citizenship be allowed?
Would the state have to issue its own currency?
Would the state deny citizenship to people living in the state who were not born in that state, but were born in another state, or another country?
What about issues of military bases in the state, and provisions for military defense? Would the rest of the US have defense treaties with the state which secedes?
Will there be tariffs on trade between the rest of the US and the state which secedes?
This is just off the top of my head. All of these and more can be negotiated. Just saying, for a time, there would be a huge can of worms opened up, to make it all work.