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

In Varjo Base (if using Varjo as the current OpenXR runtime):

1. Go to System tab
   1. Set OpenXR to ON
   2. Set Headset tracking to SteamVR (if using external SteamVR tracking) or Varjo inside-out tracking (if using the built-in headset tracking)

In SteamVR (if using SteamVR as the current OpenXR runtime):

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