Failed to unmount disk because it is currently in use.

Hello, My computer a 2011 iMac recently crashed on me. I have been able to boot it into Restore mode. When I go to Disk Utility I cannot seem to unmount the drive to erase it and try from scratch. I keep getting the error "Failed to unmount because it is currently in use."

A little more information. Yesterday I think I made matters worse in Disk Utility by hitting restore on my Macintosh HD and selected a Time Machine backup that was connected via an external HD. After the lengthly restore the computer continues to boot into Recover Mode. The Macintosh HD has now been renamed "Time Machine". When I fun First Aid on it it comes back clean. When I click on "Show All Devices" and click the drive above it and try to Erase I get a message that "Erase process has failed....couldn't unmount disk.

I feel that if I can just get this thing to erase I will be able to start making progress. At this point I do not care about losing everything on the HD. I do regular Time Machine backups so I should be able to recover what I need once I have it up and running again.

Finally I have tried reinstalling the MasOS High Sierra on it but I cannot select "Time Machine" aka the Internal Hard drive to install it too because it is a locked disc. I've been messing around in terminal and found some commands that should have done it but when I enter them they says that sudo command does not exist or something along those lines. Please help!