“If there was any constitutional issue resolved by the Civil
War, it is that there is no right to secede.”
— Antonin Scalia, late Supreme Court Justice
https://www.texastribune.org/2016/06/24/can-texas-legally-secede-united-states/
Right to declare independence is part of the charter Texas signed with the Union when it joined. The Texas Tribune may not think so but its there. Either way it will never happen so it is not worth arguing about.