Debugging extensions sometimes fails with:
> error HE0047: Failed to request XPC observation: (2). Please try again.
Retrying always works.
This is a spin-off from bug #52717.
(In reply to Rolf Bjarne Kvinge from comment #0)
> This is a spin-off from bug #52717.
Correction, it's a spin-off from bug #52710 (https://bugzilla.xamarin.com/show_bug.cgi?id=52710#c6)
I haven't been able to figure this out, nor is it very reproducible.
If it's consistently failing, one fix is to reboot the device/simulator.
Otherwise it's just a matter of trying again.