Why is the "Play in editor" button for VR preview greyed out/disabled in Unreal Engine?

"Play in editor" button for VR preview is greyed out/disabled in Unreal Engine.

In Varjo Base:

  1. Go to System tab
    1. Set OpenXR to ON.
    2. Set OpenVR to ON
    3. Set Mixed reality for OpenVR to ON
    4. Set Headset tracking to SteamVR

In SteamVR:

  1. Go to Settings - Developer
    1. Set Current OpenXR runtime to Varjo OpenXR

Please follow this documentation on how to set up your project in Unreal Engine: