Recovery server

Here is the required procedure should you have a Late-2011 iMac or later, assuming that is what you have based on your profile.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry with type and size info) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

You need a fast and reliable Internet connection. If you have Ethernet, then use it. If all you have is Wi-Fi, then be sure it is connected. The Utility Menu screen should show a Wi-Fi menubar item from which to select and log into your Wi-Fi.