How do I remove the activation lock if I can not get in contact with the previous owner?

It's hopeless. If you've followed the steps here: Turn off Find My iPhone Activation Lock and don't have any success, return your device (if you can) and get a refund. Sorry I couldn't provide a better solution... :/

The Activation Lock is an invaluable security feature that you will soon grow to appreciate once you have a working phone.

I have to tell you that It can not be bypassed and requires your knowledge of the passwords or the help of the person who originally set it up to de-activate it. Without that, the phone is of no further use.

This is Apples document which explains the feature and the whole de-activation process in more detail. https://support.apple.com/en-gb/HT201441

You don't. Without the Apple ID and password that was used to turn it on, there is no way to bypass Activation Lock.

Its a security and anti-theft mechanism, which would be rendered pointless if it could be bypassed.

If you can, return it and ask for a refund, it is useless to you.