I've bought an iPhone 7 plus and it asks for activation lock

Ask the guy what the apple id and password is. Better yet return the phone and get your money back and then buy from an authorized Apple dealer.

Read here Turn off Activation Lock - Apple Support

If you're talking about "activation lock", neither we (just users like you) nor Apple can help you, sorry.

Activation lock can be removed only by the previous owner, with the credentials that were used to activate that iPhone. Without those credentials, that device is useless to you.

Regards

Giulio

You either have the buyer take it back and return your money or take it to the police. It is useless unless the legitimate owner uses the AppleID credentials to remove the lock. There is a high probability it is stolen property, so be careful when approaching the seller.