Why can’t I download anything

Hello,

Start here:

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store

If you are trying to download an iOS update, see this:

Updating iOS: common issues and solutions

Good luck.

You can also go to the settings>general>device storage and offload any unused apps. Like this it will ask you to enter again the Apple ID and password that you used when you first downloaded the app. It is a possible scenario that you might have one Apple ID for iCloud and one for the app store.