disk1s1 device is write locked I can not boot my iMac (20" Mid 2007) running OS X 10.7.5

When I am in Single User Mode I can use fsck and when trying ti sbin/mount -uw / it says device is write locked. I am really stuck.
I wanted to upgrade to El Capitan also I can not see any startup disk
Try this:


  
  Restart the computer. Immediately after the chime hold down the (Command-Option-R) keys until a globe appears.
  The Utility Menu will appear in from 5-20 minutes. Be patient.
  Select Disk Utility and click on the Continue button.
  When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
  Click on the Erase tab in Disk Utility's main window. A panel will drop down.
  Set the partition scheme to GUID.
  Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
  Click on the Apply button, then click on the Done button when it activates.
  Quit Disk Utility and return to the Utility Menu.
  Select Reinstall OS X and click on the Continue button.
  


(Source)