You need to have a payment method,
See -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
👉 Sign in to your Apple ID account page and edit your Apple ID details.
https://appleid.apple.com
On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.
Or -> Change, add or remove Apple ID payment methods
Change, add or remove Apple ID payment methods – Apple Support (UK)
Or use Apple ID balance as payment method:
Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
What you can buy with your Apple Gift Card or Apple ID balance
What you can buy with your Apple Gift Card or Apple ID balance – Apple Support (UK)