How can I erase a lost iPhone?

If your iPhone is lost, go to iCloud.com to lock your device and mark it as lost. This will ensure that your device, account and personal information is secure.

Use Lost Mode to find your iPhone ⇢ Use Lost Mode in Find Devices on iCloud.com - Apple Support

If your iPhone was stolen or it appears on the map at a location that you don't recognize (such as an unfamiliar address), don't try to recover the device yourself. Instead, contact local law enforcement. You may be able to provide local law enforcement with your device serial number to report it stolen.

Find your device serial number ⇢ Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone location shows offline, that means that your iPhone can't connect to Apple's Find My network to provide its location. You're likely to encounter this when your iPhone has a dead battery or hasn't connected to the internet in over 24 hours.

Here’s a helpful video on how to use Find My for your information:

If your iPhone is stolen ⇢ If your iPhone or iPad is lost or stolen - Apple Support

If your iPhone, iPad, or iPod touch is lost or stolen

If your iPhone or iPad is lost or stolen - Apple Support

Find My - Official Apple Support

Find My - Official Apple Support

One app to find it all.

iCloud+ - Find My - Apple

You cannot do anything if it is offline. Any remote erase requires the device be actively connected to the internet in order to receive remote commands.