Windows ADK for Windows 8.1 (WinPE 5.0) is supported starting from build 38350
Windows Assessment and Deployment Kit (Windows ADK) for Windows 8.1 (WinPE 5.0) is supported starting from Update 2 for Acronis Backup 11.5, builds 38350 and higher.
You can see your current product build number in the menu Help -> About. If the build number is less than 38350, install the latest version of the software.