Activation Lock

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.

You can also use:

 If you forgot your Apple ID  https://support.apple.com/en-us/HT201354

If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

 If you get far enough into the process, you may be able to Use recovery mode to restore your iPhone [iDevice].

Go here if you need additional support Apple telephone support https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

For worldwide Apple ID support go here https://support.apple.com/en-us/HT204169

If you are the owner, take the phone, your sales receipt with the serial number and your ID to Apple for assistance.

If you don't have the receipt to verify that you are the owner, then you need to recover your Apple ID details or your iPhone is useless.

ASC - Late 2012 Mac mini, macOS 10.15.4; Watch, series 5 GPS/cellular, watchOS 6.2; iPad Air 2, ipadOS 13.4; iPhone 8+, iOS 13.3; TV 4th Gen, tvOS 13.4;  HomePods, audioOS 13.4 and Airport Express w/ Airplay 2