You could always read the Constitution of Texas. It’s quite clear:
“Texas is a free and independent State, subject only to the Constitution of the United States, and the maintenance of our free institutions and the perpetuity of the Union depend upon the preservation of the right of local self-government, unimpaired to all the States.”
And the Constitution of the US which Texas is subject to has no provision for a State leaving. It can’t happen without the agreement of all the other States.
Texas separatists rather like the SNP live in cloud cuckoo land.