Thanks for the reply, Ludovic.
I did try to create the partition with # 9 instead of 3, and received the same exact error. Interestingly enough, I am able to create the datastore via vSphere client, and it creates a GPT with the same exact parameters I provided in my command line.
My goal is to automate this process since I have to do it on 50 servers, so figuring out that last piece of the command line to get it working would be super helpful.
I should also specify that I am running this command after SSH-ing in to the ESXi server as root - should I be enabling a specific mode or anything like that before running my command? Should I not be running this command while SSH'ed in?