How do I recover MacBook username when locked out?

When your mac is locked, without being able to enter the required information, you must unfortunately reset that information. You can use the link below to do so. You can then assign a new user name and you will be required to get a new, different ID number. This time write that information down and save it in a secure place for the next time.

Change or reset the password of a macOS user account - Apple Support

You can use the full user name if you know it.

You can find your user name in the Users & Groups System Preferences/Settings. The user name should also the same as the name of your home user folder.

Actually if you have no other admin user accounts on that laptop and you normally have login icons for the users, then I doubt you will be able to log in even with the correct information. I have seen a macOS bug where the last user "disappears" from the login screen...when that happens it is impossible to authenticate even with correct credentials. You will likely need to restore from a backup. If Filevault is enabled, then you will need to perform a clean install followed by restoring from backup.

However, try Ronasara's suggestion first.