How to remove activation lock on my iPhone?

There is no way to take off Activation Lock without the credentials of the owner who set up the device. Activation is a theft prevention feature which cannot be bypassed. Try to return it for your money back. If that is not possible, you can recycle the device at any Apple Store. Check these links...

The All Too Common SAD Reality of Buying … - Apple Community

Activation Lock for iPhone and iPad - Apple Support

If you paid nothing for the phone, you likely got your money's worth.

That phone is going to be useless to you unless you can contact the original owner and get them to unlock the phone. The person who sold you the phone was obviously not the original owner.

More than likely, you bought a stolen phone.

Try to get your money back. If you can't, turn the phone into the police or recycle it.