Can't delete a partition in Disk Utility.

I tried to put a clean install of OS 10.12 on my Mac. I did this by making another partition in DU, then installing OS 10.12 on it using the installer you can find download on the App Store, planning to delete the original partition. But, for some reason, in Disk Utility, I can't seem to delete the old partition. For some reason, I can delete the new partition (the one I want to keep).

I attached an image showing what Disk Utility is doing....

Can't delete a partition in Disk Utility.