App Store charging for free apps on iPhone

You never have to pay for free apps.

• You may need a valid payment method and it will ask for those details if none have been provided, but you are not charged to download a free app.
• You may have a previous purchase that had not been paid yet. You will not be able to continue and use the App Store if you owe any money.

If you can’t download or update apps on your iPhone or iPad - Apple Support

What is the exact error message you're seeing? A screenshot would be helpful.