i purchased an iphone 11 from a third party. it has an activation lock. how do i remove?

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.   *

Hello ~ Contact the person/persons that you bought it from. If they cannot or will not remove the activation lock…send it back for a refund. If you cannot contact them then recycle it responsibly for we cannot help and neither will Apple. Take a look here:

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

Also:

The All Too Common

~Katana-San~