Why can't I purchase from the Apple Store?

Is this a new Apple Account? If so, you usually need to have a payment method in your account details to begin using the App Store and iTunes, plus the other Apple online content stores.

On a Mac;

• Open the System Settings app (in the  menu)

• Choose your Apple Account

• Choose Media & Purchases

• Choose View Account

• Choose Manage Payments

• Add a payment method to your account details

On an iOS/iPadOS device;

• Open the Settings app

• Choose your Apple Account

• Choose Media & Purchases

• Choose View Account

• Choose Manage Payments

• Add a payment method to your account details

You can find the acceptable payment methods for your country here;

https://support.apple.com/es/HT202631

M1 Mac mini, macOS 15.4;  Watch, series 10 GPS-only, watchOS 11.4; iPhone 16, iOS 18.4; TV 4th Gen, tvOS 18.4 HomePods, audioOS 18.4 and Airport Express w/ Airplay 2