My sister bought a macbook air 2017 last month but now she's planning to sell it cause she wanted one with better memory. She tried following the steps on https://www.google.com.ph/amp/s/www.imore.com/how-do-clean-install-macos?amp but somewhere along Step 3 number 8 she did something wrong. (She said she erased something) Now, when she restarted the MacBook, it shows a black screen with a stop symbol on it. She's already tried reinstalling OS High Sierra but it does the same thing.