iPhone 8plus

if you are encountering activation lock, the previous owner will need to help

How to remove Activation Lock - Apple Support

these are the steps that should be followed before selling an iPhone

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

When do you see this message?

When you got the phone, did it display the Hello screen in various languages?

If not, it wasn't reset properly.

If you go to the Settings app, there should be a name on the top, is it your name?

The links that I posted in my previous response should have all the information you need.

Particularly the second link, you should find a section about what the previous owner should do if they no longer have the device. If you are able to access the phone, you'd sign out of iCloud and iTunes/App Store and then erase all content and settings.

If the phone tells you that it's activation locked and the previous owner won't cooperate, it could be a stolen phone.