Suspicious lock warning on my iPhone 12

You should recognize someone is trying to Scam you. No one can lock down your phone remotely. But they can try to scare you into buying software or tools for an issue you don't have.

Don't respond to them or interact with them in any way. Block them, then delete the message.

Read this --> Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

It’s a scam.

Simply block, report and delete the message.

Important:

If you receive a suspicious email or SMS text message that looks like it’s supposed to be from Apple, please email it to reportphishing@apple.com.

Learn how to ⇢ Recognize and avoid phishing messages, phony support calls, and other scams.

Good luck! 👋🏼😉

Ignore it. It's a scam.