To simplify it:

Gold coin: you first work (dig out the gold), and then you spend the gold coin. Because your work is already done, so the gold coin is debt free -- it contains all the cost to produce it

Debt based fiat money: You first take out a loan, then you spend the money, and then you work to payback the loan

Notice the sequence here: If you first work to create money, and then spend the money, the money contains value already, no one use this money will be debt laiden

But if the money is a loan, then you need to work to payback that loan + interest, but your future income is not guaranteed due to continuously increased productivity, more and more automation/robot and less and less job opportunity, so you might run into a default

From risk point of view, the first case is 100% risk free and the second case is gambling on the future income, which is a variable no one can be sure in advance

There are more serious problem in the second case, because if all the money is created this way, due to interest, your future income must increase exponentially in order to payback those loans, means all the production and consumption must also increase exponentially, which is not possible in the long run