Sounds like you have a pending charge that has not been completed. Free apps don't charge you, as they are free.
You need to let the pending payment complete otherwise it will forever be pending.
Once it completes, you can request a refund. click here ➜ Request a refund for apps or content that you bought from Apple - Apple Support
The system is pretty full proof, if something is trying to charge you its because you started the process. Either you downloaded a free app that had a trial that renewed into paid subscription and you never cancelled it or you tried to purchase a paid app and could not complete the payment even if you did not realize it. You are now responsible for that charge and its successful completion.