I always get this message
Service unavailable - This application is temporarily over its serving quota. Please try again later.
from Google App Engine when exceed quotas, although i started setup free trial (free 300$ within 2 months) and I have enabled billing for my project. Still in the problem.
You also need to set the daily budget.
You might want to go through this Q&A: 503 Over Quota Every morning
A few other possibly related Q&As: https://stackoverflow.com/search?q=[google-app-engine]+quota+503