Cannot unmount volume?

Hi there, I have a 2013 macbook pro and need to reinstall (drive filled with ‘system stuff??)

but each time I go to disk utility I get - the erase button is light grey (so I can't select it) and it says "This operation can not be performed because you can not unmount the volume you are booted from." Does anyone have any ideas of what I should do? Thanks, John