You have a previous charge that has not been paid. This will need to be resolved before you can download any apps or even a free trial. There will not be a charge for the free trial of Apple Arcade, but any previous charges will need to be paid first.
Review your previous purchases and subscriptions here:
Subscriptions and Billing - Official Apple Support
If you are still unable to identify the charge, then contact Apple directly for the answer:
Contact Apple for support and service - Apple Support
If you do sign up for a free trial, make sure you cancel it 24 hours before the trial ends, otherwise you will be charged. That is the most common reason where users have a charge, because they forget to cancel it.