Requiring a valid payment method does not mean you bought something or owe anything or will even be charged.
It just means a payment method is required. Apple requires a valid and verifiable payment method added to all Apple IDs to make use of the iTunes an App Stores.
If it was not required before, and it is now, it generally means something happened to cause it to be required.
Normally this can happen if you downloaded a free app that had a free trial, and you forgot to cancel it. Those normally turn into paid recurring subscriptions and will attempt to charge you.
If it can't charge you for any reason, your account will enter bad standing and will not be allowed to make use of the store services which includes downloading other free apps.
You will need to allow the charge to complete successfully before anything else can be done. Once it is complete, you can request a refund for it from Apple.
To allow the charge to complete add a verifiable payment method to your account.
click here ➜ Add a payment method to your Apple ID - Apple Support
Make sure you cancel any active subscriptions you may have that charged you before requesting a refund.
click here ➜ If you want to cancel a subscription from Apple - Apple Support
and here ➜ Contact Apple Support for a refund on a cancelled subscription - Apple support
Once the charge has been successfully completed, you can remove your payment method and it should not ask for it again, unless a charge is unable to me made once again for any reason.
click here ➜ Remove a payment method from your Apple ID - Apple Support
