Free Apps are free. Apple won't charge you for a free app at all.
If there's a pending charge for something else, then it will attempt to charge it until it's successful. You won't be allowed to make use of the App Store services including downloading free apps, and until you have cleared any debt you have. It's attempting to charge you for something else you owe to Apple not for the free app you are now trying to download.
You must add a verified payment method to your account to complete the charge. Once it completes, you can request a refund if you need to.
click here ➜ View your purchase history for the App Store and other Apple media services - Apple Support