How to lock an unlocked phone?

I unlocked the phone with "My unlock" and then installed 13.0.4 without any problem. I see that the phone is still unlocked, and I want to leave it locked for security.
I tried the "fastboot oem lock" command with the fastboot from "Mi flash" and the result was a "System destroyed" screen.

I have reinstalled 13.0.4, and now I don't know how to continue, how can I lock the phone again?

Thanks