The debt must not be paid immediately, it can be paid over many decades (centuries?)Quote from Tauvros:
the U.S. government now owes more dollars than actually exist. If the U.S. government went out today and took every single penny from every single American bank, business and taxpayer, they still would not be able to pay off the national debt. And if they did that, obviously American society would stop functioning because nobody would have any money to buy or sell anything.
Besides, the Fed holds many T-Bonds, so technically this "debt" must not be paid.