On a Windows Server-based operating system, you have the operating system installed in Legacy MBR mode (PC/AT) with Trusted Platform Module (TPM) version 2.0. In this situation, you receive a message in the TPM user interface stating that "The TPM is ready for use, with reduced functionality."
On the operating systems that are listed in the “Applies To” section, TPM 2.0 is supported in UEFI mode only.
TPM 2.0 is designed to be “fully functional” in UEFI mode. Systems must be in UEFI mode with TPM enabled and secure boot configured and enabled in order to attain the security status that's described in the following TechNet article:
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Windows Server 2016, Windows Server 2012 R2 Standard, Windows Server 2012 R2 Preview, Windows Server 2012 R2 Foundation, Windows Server 2012 R2 for Embedded Systems, Windows Server 2012 R2 Essentials, Windows Server 2012 R2 Datacenter