why am i unable to download apps on my phone?

It is a requirement to have a valid payment method tied your account, EVEN for free apps. Until you put a valid payment method on your account, you will not be able download or update apps. You never have to purchase anything, so there is no cost for a free app. But without a payment method, you'll never get those free apps.

You need a valid payment method on file with Apple, even for free apps. They won't charge you for free apps but they need a payment method for ID purposes and in-app purchases.