I can't buy anything with my Apple ID

Why not? All we know is what you tell us which is not very much so far. Here are some guesses:

The support article If your Apple Account is locked, not active, or disabled - Apple Support has solutions for three frequently encountered reasons for an account to be disabled/locked/not-active:

● If a message says "your account has been disabled in the App Store and iTunes Store" or "You cannot login because your account has been locked”,if available you might be able to request reactivation by tapping Continue right from the alert. If that does not work, Apple provides a link for this specific issue. Tap or click here and choose a self-help or contact method ➞ https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 .

● If your account has been disabled for “security reasons” the solution is to reset your password as outlined in this document: If you forgot your Apple Account password - Apple Support

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple Account password - Apple Support ) to regain access to your account. More detail in —> Activation Lockout Issues - Apple Community For additional assistance from Apple Security, read: Contact Apple for support and service - Apple Support

● If your account says “Account [is] locked”, or it is inactive, you might be able to request access to your account by tapping Request Access right from the alert. If that does not work, this link (versus one taking you to the general support page) is the most direct way to get to the people helping you with this —> https://getsupport.apple.com/?PGF=PGF63005&category_id=SC0245&symptom_id=23626

or by telephone at one of the numbers in this support article --> Contact Apple for support and service - Apple Support

● According to various posts, Apple may permanently lock accounts for various reasons (very broadly outlined in section VII-B of Legal - iCloud - Apple ). If none of the above apply to you, that may be the case here. If it isn’t apparent, contact Apple and ask if that is the situation, but don’t expect them to tell you why. You, or somebody who has managed to get access to your account, are likely doing something Apple is detecting as account misuse. You will have to see if your activities could be construed–rightly or wrongly–as meeting any of Apple's conditions in the support article. You will need to create a new account and stop doing things the way you have been that may have caused the other account to be locked.

In this situation you can still use your equipment with a new account. Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Check the requirements and steps at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” Apple may typically require the original receipt* issued by Apple or an Apple Authorized Reseller. If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

If it's another reason then post here with details.