Thank you for that!
Now, can you find any legislation that was passed and signed by POTUS that explicitly states the same thing?
Every piece of legislation passed by Congress and signed by President Lincoln that dealt with the confederacy as “states of the union in rebellion” and not as a separate nation made that point: ie. Instituting a draft, blockade of southern ports, expelling southern states’ representatives and senators and replacing them with new members of Congress who were loyal to the Union, et cetera.
Is there a specific law that says states can’t secede? No, but case law has the same force as legislative law.
“If there was any constitutional issue resolved by the Civil War, it is that there is no right to secede.”— Justice Antonin Scalia