When you are creating partition in Windows Server 2012, an error message says: We couldn't create a new partition… Microsoft has an official guide on how to solve this problem in diskpart command lines, here is the link:
You can use this guide when Windows Server 2012 could not create partition
"We couldn't create a new partition server 2012" screenshot:
This guide from Microsoft can also be used to create extended partition in Windows Server 2012
Steps are alike to create GPT partition in Windows Server 2012, but some disks are MBR, so before we create GPT partition we need to Convert MBR to GPT disk, there's are two ways to do so: command prompt diskpart and Partition Expert Convert to GPT Disk Function.
Besides the command prompt diskpart method to create partition in Windows Server 2012 with different command lines, Partition Expert Server Edition can create Server Partition with ease and you can do more in this software on create partition requirements.