Missing operating system can’t install new Os The MacBook Pro was brought to me with a windows 10 bootcamp but the Mac OS portion is corrupt so it only boot to windows. When i was unable to install Mac OS Sierra on the hard drive so I removed the hard drive from the MacBook and format it in another MacBook with Mac OS extended journal format. When I insert the hard drive back to the MacBook this is the error I keep getting “Missing operating system” now I can’t install any Os on it. Any help will be appreciated
Actually this is pretty easy, if you have a network connection of some sort. Wired ethernet is generally more reliable and faster, but you need the cable and possibly an ethernet adapter, depending on the model of MacBook you have.

Boot your Mac while holding the ⌘ Command and R keys. In a minute or two you will get to the macOS recovery screen which will allow you to log into WiFi (if you are not already using ethernet) and reinstall macOS.

The system will query Apple's servers over the internet and download the latest version your Mac supports. Depending on how fast your network connection is, this could take quite a while.

It will prompt you to reboot when the installation is complete. And you will have macOS installed on your Mac.

Apple has the official details here: https://support.apple.com/en-us/HT201314