One of the most crucial parts of committing to deploying any software is making sure it meets hardware requirements....
Windows Server 2012 R2 comes with its own requirements.
Perhaps the most noteworthy and repeated hardware limitation is the need for 64-bit processors. No edition of Windows Server 2012 R2 will install or upgrade on a 32-bit computer. Legacy 32-bit servers cannot run this OS and will have to be replaced with 64-bit systems.
Second, be sure to disconnect any USB or serial port devices before starting the upgrade process. The installer must discover and identify them, and things like UPS systems can be problematic for the detection process. You can always install a conventional (VGA) monitor for installation and then reattach the USB or serial port devices afterwards.
Third, Windows Server 2012 R2 can have problems installing on the newest "4K native" drives, which forego traditional 512-byte sectors in favor of 4,096-byte (4K) sectors. Trouble usually occurs when 4K native drives are used in conjunction with a unified extensible firmware interface (UEFI) instead of traditional BIOS firmware. Setup cannot create a new partition, but you can still create a partition manually using diskpart.exe through a command prompt and then allowing the installer to continue.
Understand the software upgrade limitations
The upgrade process to Windows Server 2012 R2 also imposes some software restrictions. For example, you cannot upgrade from one language to another, you cannot upgrade from retail (free or fre) to debug (checked or chk) builds, and you cannot upgrade from pre-release (technology evaluation) versions. In these cases, a clean wipe and installation would be required. Upgrades that move from a server core to GUI mode installation or back are not possible. But once Windows Server 2012 R2 is installed, you can switch between modes at your discretion.
Distilling the installation requirements for Windows Server 2012 R2
Make sense of Windows Server licenses
Prepare for the upgrade to Windows Server 2012 R2