Ensure you have run Minecraft 1.11 at least once so the base files exist.
Wurst v6.6 for Minecraft 1.11 was a significant maintenance and feature update released in early 2017, shortly after the transition to the Minecraft 1.11 "Exploration Update." This version of the Wurst Client was part of the stable v6 series, which focused on refining the user interface and expanding combat and utility features.
Features the classic ClickGUI (typically opened with the Right Shift key) and the TabGUI for quick, keyboard-based navigation.