Locked Mac find my Mac
You need to visit an Apple Store (Genius Bar appointment) to get it unlocked.
https://support.apple.com/en-us/HT204756

How do I unlock my Mac?
Once you locate your Mac, you can unlock it by
entering your passcode on the lock screen. If you forget your
passcode, you'll need to take your Mac to an Apple Authorized Service
Provider along with your proof of purchase.