How to restore MacBook Pro to factory settings? Prohibited sign on boot Because I am selling my old MacBook Pro (13-inch, 2011) and, for this reason, restore it to the factory settings. I have followed the steps for this. First the discs were erased which only partially succeeded. In the next step "reinstall macOS" I now get a gray screen and I can't get out (even after I turned off the laptop). 

How can I proceed from here?
These links may help: 

1) https://mackeeper.com/blog/post/473-step-by-step-guide-to-reset-mac-to-factory-settings/

2) https://www.macworld.co.uk/how-to/mac/reset-mac-3494564/

3) https://macpaw.com/how-to/factory-reset-mac