I\'m facing problem with Google push notifications (for drive). I use service account which works perfectly for all other drive operations except drive changes watch.
For me, as I put in a comment above,
The Domain Verification was not saving in google developer console (refresh the page and it was gone). The problem ultimately ended up being I was logged in as two google accounts, my gmail account and my company account. Adding the domain verification seemed to get confused about the account and not save the domain settings.
Try logging in using a different browser or incognito session if you use multiple google accounts.