Apple - Deleting a partition in Disk Utility fails. (Error: -69879)

Never mind - problem solved.

Turns out that the whole drive's partition table cannot be modified if one partition has encryption on, since CoreStorage is on. (The one that I had on was the encrypt feature for Time Machine.)

I had this same issue a few days ago, Go back into disk Utility and use the erase option rather than the Partition option. So select the drive you want to get rid off and select erase. Hope this helps