My eSim is not working and give a notification of “carrier lock”

Click on the link to read Apple's support article on how to unlock your iPhone for use with a different carrier. - How to unlock your iPhone for use with a different carrier - Apple Support - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Tap Settings > General > About

Scroll down and look for Carrier or Network to see if information appears there.

If someone else used the phone before you, they would know who they were paying the cellular bills to each month.