Erase function in Disk Utility is not functioning

I have a MacBook Pro9 which I am trading in via Apple GiveBack. Before I can ship the MacBook I need to erase the hard drive and do a clean install of High Sierra. However, the Erase button is greyed out " because you cannot unmount the volume you are booted from". How do I fix this? I need to get this resolved quickly as the trade-in value is valid for just 14 days from now.