Why can’t I download apps not even free one’s?

Unless you have a None option in the payment methods of your account details, you must have a valid payment method, even for free apps.

An alternative to a bank card is to buy & redeem an iTunes/App Store gift card.

Do you have a payment method in your account details?

M1 Mac mini, macOS 11.4; Watch, series 5 GPS/cellular, watchOS 7.5; iPad Pro 2nd Gen, iPadOS 14.6; iPad Air 2, iPadOS 14.6; iPhone 12 Pro, iOS 14.6; TV 4th Gen, tvOS 14.6  HomePods, audioOS 14.6 and Airport Express w/ Airplay 2