I got an iPhone XR from Facebook market place and it has activation lock

Hello ~ That is the activation lock and the device was not properly prepared for sale. If the person/persons that you bought the iPhone from cannot or will not remove it …the iPhone is basically useless to you. If they cannot/will not remove it…return it and get your money back. We cannot help remove it nor can Apple. Take a look here….

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also…

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

You will need the previous owner's assistance. See: Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If the previous owner won't help, there is no way to bypass activation lock. If you can't get a refund, Facebook may offer some protection: https://www.facebook.com/help/228307904608701

Activation Lock is an essential tool to protect your data if your device is stolen. The only way it can be removed is by the owner of the device entering their ID and Password. Without that information the device will be of no use. If there was a way to bypass Activation Lock, it would defeat the whole purpose of having it and bad actors would be able to access your private information.

The only way to purchase a device from a third party is to physically startup up the device and it should be on the "Hello" screen to set it up as a new phone. These are the steps the person you bought it from needs to do before another person can use it (specifically the Remove your Personal Information section).

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