Replace the Digitizer/LCD Assembly and it should fix the issue. If a new Screen assembly doesn't work then could be a problem with the Logic Board. Or make sure the Cables are plugged in right for the digi on the current screen.
There is no code , i think the seller was misinformed, but there is a bobby trap at least from what I am hearing on the iphone X that will detect if the face ID is removed out of the phone making repairs a little bit more complex. I'm assuming it has some kinda kill switch when removed and tried to be put back in it shuts the phone down to prevent someone from putting in another face id and getting a user's info. So they do it for security reasons. Either way I am not opening any Iphone X till I find more out, but anything lower then this I have repaired and heard nothing about a code. So maybe they also heard this and just put their spin on it .
You could try a new screen assembly , but if it still is doing it then I agree with nicolay.94 it's brobably the touch disease issue which was common on this phone and would require soldering o fix the logic board
Most likely the case. Take it out of the case and try it . If it works then it's your case. IF not take it back under 1 year manufactures warranty and let the company know it hasn't worked. They should replaced the phone.
bad lcd OR lcd not plugged in correctly , if you replaced screen . If it's a newer screen I would order another replaced send that back as defective, Something with the LCD is bad whether it's the connection on board or the screen itself.