I want to lock and erase my stolen phone

If Find My was enabled on the device, and it is online, use lost mode to lock it. If you erase it, you will no longer be able to remotely track it.

See -> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If the device did not have a screen lock passcode already, then anyone can simply open it and disable Find My. Note too that anyone can also just turn it off, or erase it with iTunes.

Did you turn on Find My?

How did they get past Face ID and your passcode?

Then there is nothing you can do remotely with it. All you can do now is report the theft to the police, and contact your carrier to see if they can blacklist it.