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:
https://developer.varjo.com/docs/unreal/ue5/getting-started-with-unreal5