Hey there! Sorry for the troubles!
Peter_Watt recommended the right article to follow. I’d suggest if you’ve had that payment method for a while and suddenly stops working, even for a free app to check out your payment and billing history:
See your purchase history for the iTunes and App Store:
https://support.apple.com/en-us/HT204088
Have you recently moved, changed your payment method, or bank?
The payment history and subscriptions may be able to shed some light, of there is an open subscription that requires payment before the App Store can be used, or a new payment method.
Also sometimes it may just not work as expected.
If so, I’d make sure the date and time, as well as the language and region settings are set correctly in Settings > General.
If you are downloading from a different Country than your payment method, you may have to change the billing info to the current Country.
I have seen however a glitch every now and then, where you have a valid payment method, but need to change the billing address to a new address, but doesn’t allow for it in your iPhone or Mac settings, in such a case I’d try updating your billing address at applied.apple.com saving the changes and trying again.
Otherwise in iTunes on a Mac or PC click either Store or Account > View Apple ID, you can try to update the information there as well.
Testing by purchasing (even for free) from another device with the same Apple ID is a good step, just to rule out an account, device or network issue),
Sometimes the updated terms and conditions just need to be accepted, and won’t work when updating your information from one device or browser but work in another.
It is also possible that your Apple ID is locked from being used for purchases due to a billing issue, in such a case, if updating the current billing information doesn’t fix it, you may have to contact Apple directly to fix it, (Just make sure you can verify your identity by having access to your Apple ID).
This is a very complete article provided by Apple, Hope this can help further!:
Subscriptions and Billing:
https://support.apple.com/billing
