If you are using the internal drive as the boot volume, you can not erase it. It is impossible to erase the boot volume - the running OS will not suicide.
If that is the case, the solution is to boot the machine to a different volume - typically an installation disk, but can also be a different hard drive such as an external one. Once you are booted to a volume other than the one you want to erase, you can use Disk Utility to erase that volume.
Thanks for the quikc response. I booted up in the Recovery Disk I believe. I held option at boot up and chose the recovery disk option. Is that the correct method?
I may have figured out what I was doing wrong. I was clicking on the parent hard drive reading "1 TB WDC WD1001Fals..." Now I've clicked on the next level hard drive "Macintosh HD" and I believe it is working.
Looks like 5 hours to erase...