How to remove an activation lock on iPhone 6 plus?

Contact the seller for removal of Activation Lock or for a refund, or—if the seller has disappeared—recycle this iPad.

We can’t help. Without original receipts or similar, Apple won’t. Without the assistance of the holder of that Apple ID, and without proof of ownership sufficient to convince to Apple, this iPhone will be forever useless.

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

For recovering access to your own Apple ID, if you are the holder of the Apple ID associated with the Activation Lock:

If you forgot your Apple ID password - Apple Support

If you forgot your Apple ID - Apple Support

Apple’s I Forgot Service

How to use account recovery when you can’t reset your Apple ID password - Apple Support

Contact Apple for help with Apple ID account security - Apple Support

If you are the heir:

How to request access to a deceased family member's Apple accounts - Apple Support

Are you the original owner? If yes, you need to enter your Apple ID and Password to unlock the phone.

If you are not the original owner, you will need to find the original owner and convince them to give you their Apple ID and Password. They may be a lot less than thrilled to provide you with this information if their phone was lost or stolen.

Otherwise, the phone is useless. Turn it into the police or drop it off at the recycling place.

You can’t remove it without the credentials for the account used to lock it.