I have not seen a definitive solution for your problem. So, I suggest you use my search link in trying to solve your problem: cannot be used to unlock this iPhone. The document found seem to point to activation lock related to a second-hand acquire iPhone. See below.
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See Turn off Activation Lock https://support.apple.com/en-us/HT201441.
If you acquired your device second hand, you usually will need to obtain the needed password from the prior owner or have the prior owner sign out of iCloud; erase the device; and remove it from their device list. If unsuccessful, in the rare case your device came activation locked from an authorized Apple seller, then contact Apple Support and present your original purchase receipt for your device with the device’s serial number on it. You’ll need a photo ID too. If you can do none of the above, then the device is useless to you.
You can use the suggested support options below if you have the needed receipt. Alternatively, if an Apple Store is open in your area, then you can go there to deal with your problem. In this case, you can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if you want.
If you want to contact Apple Support to deal with your problem, see:
Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient. Sometime this site is not fully functional.
Apple ID Support https://support.apple.com/apple-id
Apple Support Twitter https://twitter.com/AppleSupport There may be a wait of an hour or so.
Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best one.