I can't download anything from the app store

This is unfortunately a long list of things to check but you have provided too little information for us to propose a specific solution. When you start your first post describe what happens when you try to download, and the exact wording of any error messages. This would greatly aid us in helping you, and keeping this list shorter and more directed. In the meantime review these suggestions and documents if you think they apply:

Read this article to make sure you know how to download apps and games on your iPhone or iPad

Download apps and games on your iPhone or iPad - Apple Support

Read this document if you can’t download or update apps on your iPhone or iPad --> If you can’t download or update apps - Apple Support

If the first 5 don't help, use the 6th.

If you see a message that says "Your account has been disabled in the App Store and iTunes", follow the directions in this document --> If a message says 'Your account has been disabled in the App Store and iTunes' - Apple Support

If a feature on your device is not working, it may have been disabled. See how to use Screen Time on an iPhone, iPad, or iPod touch by clicking here --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support Go to: Content & Privacy Restrictions > iTunes & App Store Purchases and allow adding and deleting apps.

Learn what to do if you can't redownload an item or if you don't see the item in your purchases.

If there's an issue when you redownload apps, movies, or other content from Apple - Apple Support

- Items bought with another Apple ID, and about Family Sharing.

- Incompatible apps.

- Hidden items.

Other considerations:

Do you have the requisite system version for the app you are trying to download?

Do you have the same problem when using a different Wi-Fi network?

Does restarting your device make any change?

Can you download apps normally on another device?

Did you try restarting your device?

Resolve issues between iTunes and third-party security software - Resolve issues between iCloud for Windows or iTunes and third-party security software - Apple Support - You can’t connect to iTunes Store to buy or download content.

Do you need to add a payment method? This document explains how to add a payment method to your Apple ID on an Apple mobile device, a Mac, or a PC --> Add a payment method to your Apple ID - Apple Support

Check Apple's system status to see if a server is having problems --> Apple - Support - System Status

If you can't connect or sign in to iCloud, see if this document has a solution --> If you can't connect or sign in to iCloud - Apple Support

If you can't connect to the App Store, iTunes Store, or Apple Books, read this support article --> If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

- If none of the suggestions work, try the last one: "If you’ve tried all of the above steps and you still can’t connect, contact Apple Support."

Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232

If your payment method is declined in the App Store or iTunes Store, see -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

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How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

See the following Apple articles:

Payment methods you can use with your Apple ID

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

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you need to change or update your Apple ID payment method

If you need to change or update your Apple ID payment method - Apple Support

• If you want to change your card, you need to add first the new card, then remove the old card.
• or if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.

Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, you need to add first the new card, then remove the old card.
• or if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.