Are there any states that don't have debt? I've not been able to find any states that don't have debt to other states.  This, to me at least, seems really strange.  Are there any nation states that are not in debt to other states, if so which ones, and if not why can't everyone just settle the debts?

(Wasn't sure if I should ask this on the Politics Stack Exchange or here, please let me know if this is out of scope and I'll ask there.)
The classic answer here would be Libya and Brunei, but I think Libya now has debt.

Brunei is a strange case in that it uses a joint currency with Singapore dollar, controlled by the monetary authority of Singapore, so in effect you can use Singapore debt as a substitute for Brunei dollar investment.

Not having any debt, and having a free currency is generally a bad idea, since you cannot control interest rates. Furthermore, if you can afford not to have debt, then people will be begging to lend to you at such favorable rates it would be foolish not to borrow a bit.

On the other hand there are many counties with no net debt. Singapore and Norway don't need to borrow but do so to provide bonds for the financial markets. They then invest the money they borrow. Norway can borrow so cheaply they can buy US treasuries and make a profit.