why can puerto rico split from the us but a state cant?

Puerto Rico has had several votes on whether to remain what it is now, leave entirely or join the US. The leave entirely group never even got serious consideration. In the last vote they voted to join the USA. Now it's up to Congress to decide how they want to handle it.

Incidentally, you don't realize how INCREDIBLY stupid the South was to secede. There were still people ALIVE at the time who remembered how bad the original Confederation was, how ineffective & ludicrous it became in just a few short years. Lincoln only wanted to buy out the slaves over a period of 35+ years, exactly the way Britain had successfully done a a couple of decades before. So the slave owners lost their slaves for no compensation & the Negroes were free 35 years sooner. Even before the Civil War ended incidents of each state trying to decide how much it would participate were happening -- things like 1 of the textile states making uniforms only for its own men. Had they won, the Confedracy either would have become a totalitarian dictatorship, a collection of totalitarianisms, or rejoined a much stronger USA piecemeal. Manifest Destiny would have become more important to the USA, which would have widened the gap in force between the USA & a putative CSA government, assuming it could keep together.

If you legislate secession into a non-totalitarian government, it is only a matter of time before it is used by the parts. And you've MADE IT LEGAL.

the U.S. can allow it to split because it is not a state. States have to adopt the constitution which does not accept states to leave?

I've only heard of them voting to become a state. I can't think of any us possessions/territories that were made independent. not that it couldn't have happened, just I can't recall/think of.

Puerto Rico is only a territory so I think it is easier for them to leave.

Its not a state