Debt can absolutely be issued with Bitcoin. What's to stop someone from saying, here I'll give you 1 BTC today if you give it back to me tomorrow? The reason you've probably heard that debt cannot be issued in Bitcoin is because debt cannot be issued in a 'safe' way. In other words if I give someone 1 BTC today, I can't be sure that they will give it back to me tomorrow. But that's just the nature of debt and has nothing to do with Bitcoin.