Hey Huy,
You should post platform issues to the
Public Issue Tracker for App Engine, generally, but I'll be happy to take a look at this issue here first until we can make a thread there.
Could you email me directly a screenshot of what your Developers' Console looks like in the "App Engine > Settings > Custom domains" view? My email, if you can't see it, is
pay...@google.com.
Cheers,
Nick
Cloud Platform Community Support