The Lakota tribe cannot secede from the United States because they were never a State. It is a recognized sovereign Nation with a treaty. Like any other contract, treaties can be terminated.
Good luck getting Chief Tapdancer to give you a straight answer.
They have a higher order of right to secede than any state does, because they were a sovereign nation before joining the U.S. They actually existed as a nation even before the U.S. itself existed on this land. They actually precede the U.S. on this land.
So, I would say that they have a higher order of right to secede than any state does...