Huh, that's a variable that should be defined upon the game starting up and there's nothing that should delete it...
Have you tried closing and relaunching the game? I can't reproduce this and chances are that this is one of those exceedingly rare bugs where Ren'Py just forgets to define something.
If that doesn't work (and you're on PC), you can open up the console with SHIFT + O, input "zk_scarred = False" without quotes, then close out of the console by inputting "exit" without quotes. If you're on Android, that method won't work but there is a new game checkpoint you can start at right at the start of the Athlima Summit.