IBKR always says I don't have enough cash settled to make a trade. WHY?

if you have 10’000 CHF and you want to have 0 CHF, you cannot achieve this with USD.CHF. You have to sell CHF.USD…

Just a theory, but does not IB charge (some) fees in account currency? So if you had 0 CHF, then maybe it was not possible to charge the fees?