I bought a MacBook whose iCloud is locked and owner doesn't know the credentials

Ask that someone to properly prepare that Mac for sale. See below. It cannot be used in its present condition.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Return it to the seller for a refund - if you think it is safe to do so.

Activation Lock is an anti-theft measure. We cannot remove it for you, and Apple will not, absent proof of original purchase that is satisfactory to them. Without the help of the owner (who you say has forgotten their own credentials), that Mac will remain a “brick.”

Kimberley2005 wrote:

I don’t wanna use it and I can’t get in it just like the previous owner I wanna delete it from the MacBook but without Apple ID or password I can’t

If you want to use the MacBook, here's what has to happen (based on what I know of how the process works with iPhones).

The original owner has to be

• Willing to clear the Activation Lock
• Able to remember their Apple ID and password; or to get Apple's help in clearing the Activation Lock
• Willing to follow the steps in What to do before you sell, give away, trade in, or recycle your Mac - Apple Support once the Mac is unlocked
• Willing to transfer the MacBook back to you afterwards

For Apple to help clear Activation Lock, they will need original proof of purchase that's satisfactory to them. That will probably have to be from Apple or an Apple Authorized dealer, and to show the serial number of the Mac that the original owner wants them to unlock, if there is to be any chance that Apple will agree to remove the lock.

⛔️ Return it for a refund A.S.A.P. ⚠️

You don’t know if it was stolen and don’t want to be held responsible.

There's nothing to delete, it's all web site based. Just stop trying to access it. Make an appointment with Apple Store nearest to you.

Ordinarily, you shouldn't have access to a previous owner's iCloud account. You need to establish your own iCloud account and not trying to use the previous owner's account. An iCloud account belongs to the person that created it. It's not something that's part of the computer.