How to take off the activation lock on iPhone 7 plus?

Without the help of the previous owner, you will NEVER be able to use this phone. Either get the previous owner to remove the lock or return the phone for refund or recycle it. Apple won't help you and we can't help you.

See this link --> The All Too Common SAD Reality of Buying … - Apple Community

If the previous owner can not, or will not remove the lock, the phone is useless to you. Activation lock can not be bypassed.