On the first debt payment? Or on a later one? It starts at 250 credits, and doubles every due date.
If you can't afford to pay the debt payment, then you've lost the game - I just haven't added a fail state to the game yet per se, so I made it so players can continue playing if they are able to recover after going into the negative. If you can't really recover, then you can start a new game.
If you can't afford to bid on any planets, then you can still mine one for free - but it's a very low level planet.