If you're being prompted for your own Apple ID, use Apple's iForgot service or contact their Account Security team.
If not, but you're the device's original owner, take the device and its original purchase receipt to a physical Apple Store.
If not, but you're able to contact the person who set the lock, send them this link and tell them to follow its instructions.
If none of the above applies, you can't activate the device but may be able to get it refunded. Apple won't remove the lock for anyone other than the original owner(assuming they're alive.)
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