how to fix error 1015

You don't. That error is usually a sign that the phone was hacked/jailbroken. Was it?

Look under the More Like This section on the right of this post, but generally this means that your phone has been jailbroken and there is nothing you can do.

You have an unusable brick and might need to buy a new phone.

As FelipeV noted, that Error message only occurs when an iPhone has been "jailbroken." Apple does not offer any support for jailbroken devices, and we users in these Forums are not allowed under the Terms of Service to discuss any resulting problems. You will have to look elsewhere for help.