I can’t download anything with my Apple ID

You may need to have a payment method on file, even if the app you want to download is free.

Check which payment method you have on file 

Sign in to your Apple ID account page and edit / update your Apple ID details.

https://appleid.apple.com

On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

See also the following Apple articles:

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)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

What happens when you try? Error message(s)? More information would help.