Can't divide disk by using Disk Utility
The problem is you didn't select the disk properly. You see how you selected "WD"? You need to select the item above it, where it says "WD My Passport...". Then you'll be able to partition it.

Edit (from Tetsujin): You also need to make sure that the drive is formatted as Mac OS Journaled (also known as HFS+) with a GUID partition scheme.