Why can't debt be paid or traded?
There are no resources in adequate supply to pay for it. Trading it has the same problem.
Even if you went around the world trading debt until every nation owned their own debt...those nations...just about all of them...have no way to cover it.
Debt must be paid for with wealth. That means there must be resources to cover that debt. Trying to pay for it with fiat “money”, just causes hyperinflation in any...and probably every...economy.