That must be a reference to Article I, Section 10, Clause 1. I’m not sure if that is a reference to secession or simply a prohibition of a group a States forming a sub-country within this country as a layer of government higher than those individual States, but lower than the federal government.
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
It is clearly intended to keep the individual States from conducting their own foreign policy.