Explain how the median debt can be zero?

:  If there are more Americans who have no credit card debt than there are Americans with credit card debt (regardless of how much), then the median will be zero. The median represents the point of separation of two halves of a data sample, and does not take into account what the values are on either end. For example, if five households had the following debts: 

0, 0, 0, 5000, 10000 

the median would be 0 as it is the center value. 

Note that this is just theoretically why your scenario could be true. I have no idea what the actual statistics are on American credit card debt.

This depends on what other numbers there are in the equation.