This is because the processor in your Microsoft Surface is not a regular PC-type processor, although it is still running Windows 8. You will have to download Office 2013 which is compatible.
“Microsoft Surface is a series of tablets designed and marketed by Microsoft. It was announced on June 18, 2012, by Microsoft CEO Steve Ballmer at Milk Studios in Los Angeles.The Surface comes in two versions: one with Windows RT and another with Windows 8 Pro. The Windows RT model uses an ARM CPU, while the Windows 8 Pro model will use an Intel CPU. Both models are able to install new applications via the Windows Store, however only the Windows 8 Pro model allows the installation of traditional third-party desktop programs.”