I purchased a second hand iPhone and I can't unlock it

You need the Apple ID and password that was used when the device was first set up. If you don’t know that, you can’t remove the Activation Lock.

The previous owner needs to go to https://www.icloud.com/find/ and erase the device, then remove it from their account. If they won't do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.   *

See if you can return the phone for a refund.

I purchased a second hand iPhone and it won't let me unlock the phone without the original owners Apple ID and password so what do I do now

Unless you can get a hold of the original owner, return the phone for a refund. As it stands now, you should not have paid more than zero for the phone, since that is what it is worth until the original owner unlocks the phone.