Factory reset
I restored my iMac to factory settings but now I am unable install the Operating system. It does not show any disc option to install.
iMac 27″, macOS 10.14
If as your profile says you are on macOS Mojave 10.14 then you need to erase the Disk correctly.
From your Recovery HD open Disk Utility now click on View in the menubar and select
Show All Devices.
The left hand column will now show the Disk (Apple SSD.... or similar) and
the Containers and Volumes indented underneath.
To erase the disk correctly highlight the Disk and click on Erase.
Give the Disk a name.
Depending on which OS you are going to reinstall
Format: APFS for macOS High Sierra or newer,
MacOS Extended (Journaled for macOS Sierra or older.
Scheme: GUID Partition Map.
Click Erase.
When Done quit Disk Utility.