欠债 is a separable verb (离合词) meaning 'to owe a debt'. It splits to insert measure words, amounts, or durations between 欠 and 债, e.g., 欠了一笔债 ('owe a sum of debt') or 欠了很多债 ('owe a lot of debt'). You can also add a person after 欠 with 给: 欠给银行一笔债. The object of the verb (the debt) is already built into the compound, so you don't add another object after 债 — specify the amount or type instead.