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See Turn off Activation Lock https://support.apple.com/en-us/HT201441 “If you erase your device without signing out of iCloud first, it might still be protected by Activation Lock. Whether you erased your own device or … [got an erased] one from someone else, you can only turn off Activation Lock by entering the correct Apple ID and password.” The correct Apple ID and password is the combination allowed by the rightful owner of the device. 

So, if you forgot the correct Apple ID or Apple ID password, use Having trouble signing in/iForgot  https://iforgot.apple.com/password/verify/appleid to identify either or both.

Activation Lock or did you forget the passcode?

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support