Seems like an emerging known issue:
https://www.reddit.com/r/WindowsHelp/comments/1wab4w6/game_bar_capture_not_working_greyed_out_fix/
- Press Windows key + R to open Run.
- Type
regedit, then press Enter. If prompted, choose Yes.
- In Registry Editor, go to:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\GameDVR
- In the right pane, find AppCaptureEnabled.
- To take a screenshot, press Windows key + Shift + S, then drag over the Registry Editor window or the value you want to show.
- Click the screenshot preview to save or copy it.
Paste it in the next reply.
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