Appengine stuck on "Updating service [default]..."
When you get this error, you can take a look at crash.log in the Cloud Console Logs Viewer (Logging -> Logs) for your specific service / version which will usually tell you exactly what happened. In my case when I reproduced the same error, crash.log shows me the output of NPM which tells me I have a 'SyntaxError: Unexpected identifier'.
YMMV of course, but this can tell you if the issue is related to your application code or if there is something more sinister going on.
The answer for me was different... The version started to get created but never finished the creation process. This is scenario is visible if you go into the logs for your AppEngine service (check all options to see ALL logs) and you may notice "Container called exit(1)."
In my case I found that running "gcloud init" to reset my credentials helped. Hope this helps someone else.
Scratch that... I found that the Cloud Build API was just WAY behind / slow. This has been happening for the last couple of hours. I just noticed 4 versions pop into my dashboard all at once from prior hours in the evening. Apparently there is no way to cancel prior deployments? So ironically once Google's build service gets behind everyone probably starts spamming / retrying their deployments and the issue gets worse... so bad.