How do I unlock a second-hand iPhone 6 with owner's passcode?

Your iPhone is Activation Locked. It can never be used without the previous owner’s assistance. Get a refund if possible.

How to remove Activation Lock - Apple Support

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

rukshan18 wrote:

I BAY IKMAN.LK AND HE NOT GIVING THE MANY AND Passcode

[Re-Titled by Moderator]

Original Title: I AM BAY SECOND HAND IPHONE 6 OWNER LOCKED THE PHONE PLZ HELP MI UNLOCK MY PHONE HE Doesn't RESPOND

You've been scammed and you wasted your money. Apple will NOT help you unlock the phone. If you can't get your money back, which is unlikely, recycle the phone responsibly as it is utterly useless to you. Read the link sberman posted about The All Too Common Sad Reality of Buying Used Devices.

Learn from this costly mistake.

1. (Passcode-)locked iPhones can only be unlocked by the previous owner. If one would buy a second hand iPhone from a trusted source, then it is vital that the iPhone has been unlinked from the previously used Apple Account, and that the iPhone has been reset to factory settings. No one can do that for you, except the previous owner. What to do before you sell, give away, or trade in your iPhone - Apple Support
2. SIM-locked iPhones can only be unlocked from the mobile provider lock, by that same mobile provider on request of the phone’s owner that had the locked mobile contract. Apple can’t assist with that. How to unlock your iPhone for use with a different carrier - Apple Support

An iPhone 6 is quite old (2014) and cannot run modern software anymore (iOS 12.x at maximum). It will be quite limited in what it can do, and what apps it will support, even if you had it unlocked.