The iPhone is locked by the owner. I do not remember the password.

You can contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

See also -> Keep your receipt after you buy an Apple device

Keep your receipt after you buy an Apple device – Apple Support (UK)

Yes but the proof of purchase is gone apparently… The other thing that perhaps you can do is get into icloud.com or https://appleid.apple.com see if you can unlock it from there… Or indeed try to reach support see how they can help without the proof of purchase

You will need to recover the password for the Apple Account that the phone is tied to. Without that the phone is useless.

What error?

Terribly sorry… Can you please translate that? Cyrillic is not my forte :)

The proof of payment proves that the iPhone is yours.