It won’t let me buy anything

Unfortunately you have not provided sufficient information for us to propose a specific solution. Describing what happens when you try, and the exact wording of any error messages, would greatly aid us in helping you. In the meantime review these documents if you think they apply:

Click on this link "If you can’t download or update apps on your iPhone or iPad " --> https://support.apple.com/HT207165

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

Considerations:

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?

If instead you see a message that says "Your account has been disabled in the App Store and iTunes" click on this --> https://support.apple.com/HT208856

Resolve issues between iTunes and third-party security software - https://support.apple.com/HT201413 - You can’t connect to iTunes Store to buy or download content.

Apple Recommended article by ASC contributor Urquhart1244 --> https://discussions.apple.com/thread/8434425 with various suggestions about network, payment and log-in ussues.

"If you can't connect to the App Store, iTunes Store, or Apple Books" click here --> https://support.apple.com/HT201400

- 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."