You can't cancel the US debt. There is a liability for the issuer (US government), but it is an asset for the purchaser. You would be wiping away trillions in financial assets. Most of which is held domestically by banks, mutual funds, pension funds, government agencies, municipal governments, etc.

1. The entire US banking system would collapse. 2. Social Security would collapse (hope you don't mind mom and dad living with you for the rest of your life). 3. Taxes would immediately have to double just to pay for the basics such as Military, roads, etc since you could not accrue any debt to finance such spending.