support.apple.com/kb/TS4515

The page you give the URL off basically says it all. Did you buy the device second-hand (used)? If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it).

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.

If on the other hand you bought it new, and the ID it's asking for is yours but you've forgotten it, go to https://iforgot.apple.com to retrieve the ID and reset the password. See also If you forgot your Apple ID - Apple Support

Failing that, take the device, the original purchase documents, and some ID to an actual Apple Store (make an appointment) and ask if they can unlock it.

You’re asking here because you can’t contact the seller to unlock the device or obtain a refund. Which usually means you bought a lost or stolen iOS device. If this is the common case, you likely got ripped off. Chat with the local police. There’s deliberately no way to remove that activation lock by anyone other than Apple or the original owner, and that expressly to make these devices worth less or worthless to potential buyers.