help apple iphone 6S plus Lost mode

No you're right. Your iPhone is activation locked, which means you need to enter the Apple ID and Password you used to set up the iPhone. The iPhone has already been erased, so your content is gone, but the ONLY way back in is to enter the ID and Password.

What was the email address you had when you got this iPhone? That is most likely your Apple ID. Do you know your Apple ID password? It would be the password you used to buy apps from the App Store, etc.

All you have to do to get back in a phone which was put in lost mode is enter your 4 or 6 digit passcode.

If you have the original purchase receipt (you may not need it since you say your know your Apple ID, but it would help if you do have it), take it and the iPhone to your Apple Store for directly help from Apple.

If you are sure you know the right Apple ID, use this link to ask Apple to reset the Apple ID password for you.

If you forgot your Apple ID password - Apple Support