TBH, you are not in debt. You didn’t get the shares at all in the first place in reality. It’s just that their app shows that you got the shares.
Now w.r.t how you are in debt in their app – they would have shown you that they are allotting you more shares than you applied for and would mention that the money is got stuck there and if you want to get it, you need to pay for the rest of the shares and then can withdraw as a lot.