I can’t download apps on my iPhone because of a billing issue

This is a user to user only forum. If you have a billing issue, you need to get support from Apple directly.

If you see a message your account has been disabled in the App Store or iTunes, click the link inside this link to ask for help reactivating your account --> If a message says 'Your account has been disabled in the App Store and iTunes' - Apple Support

Do you have a valid payment method on file? If the card you may be using expired you need to update it.