Activation lock appeared after I restore the iphone7 I bought second hand

"Activation Lock for iPhone, iPad, and iPod touch" is a security feature, one aspect of which is deterring re-selling of stolen devices. To learn more click here --> http://support.apple.com/HT201365

An activation lock can only be removed using information for the account that applied the lock. If you know this information follow the onscreen steps to enter the Apple ID and password used to set up the device, or enter the device passcode.

Click on this link to learn 'How to remove Activation Lock' --> https://support.apple.com/HT201441

If you do not know the required information and you are not the original owner, get the owner of the account to remove the lock or return the device for a refund or recycle it. The device is otherwise useless to you. If the owner is unknown and has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

Only the owner who set up the phone can remove activation lock. Apple won't do it. We can't do it. If the owner doesn't remove the activation lock, you are the proud owner of a useless brick, which you'll NEVER be able to use.

Either get them to remove Activation Lock or get your money back. If they are or can't do either, recycle the phone as it's now utterly useless to you.

The owner has options to recovery their account from Apple.

Get your money back.