Why can't I download free apps when I owe money

Because you cannot use the App Store services if you owe it money. Services include the usage of bandwidth to download an App.

Until your account is in good standing you cannot download anything. Paid or free.

how do i pay them back??

How to download app free

HHelp

Whether it is free or not does not matter. As Carolyn said, your account is not in good standing and you need to rectify the money you owe before you can use the store again. Those are Apple's rules. Pay your bill.

Follow the steps in this support document to complete payment:

If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - Apple Sup…

Hi,

The balance due must be paid prior to downloading free apps.

If you don't owe iTunes anything then click/tap on the app's 'get' button to download it. If you owe iTunes money pay what you owe and you will then be able to download items

See above