You don't have to pay to download free apps. You may still need to have a credit card on file in order to do that. See step 2 here:
If you can’t download or update apps on your iPhone or iPad - Apple Support
It does appear you have a previous purchase that has not been paid and that will be processed when you add a card. You can view your Purchase History and any amount in red is a pending charging that still needs paid.
View your purchase history for the App Store and other Apple media services - Apple Support
If when you add a card you see a message that there was a problem with a previous purchase, then look here:
If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support
You will not be able to use the App Store to download any apps even if they are free, until you have resolved that issue.