erase key is grey out
i try to manage my macbook air - OSX 11.6 storage but there is 'OTHER" items took up 80G from my disk. My questions are:
1) trying to manage storage manually, I already removed all apps and programs I don't need, but I don't know what are in "other" and how to open it to delete unwanted app/programs
2) tried using DISK UTILITY - to restore from manufacturer default but the ERASE BUTTON is GREY OUT . Please tell me why and how to fix/use that erase button to restore my mac
MacBook Air 13", OS X 10.11