Hello, I could use a little more information (e.g. stack trace, debugging log, screenshots, Google Cloud SDK version) to provide guidance here. However, I'll suggest a few reasons you encountered this along with some solutions I've seen:
1) you ran 'gcloud app deploy' and you are using a deprecated version of the SDK (run 'gcloud version' to check)
- run 'gcloud components update' to upgrade to the latest Google Cloud SDK [1]. At the time of this post the latest Google Cloud SDK version is 217.0 [2]
- rerun 'gcloud app deploy --verbosity=debug' command (assuming you used this) to deploy your app [3]. Take note of the verbosity flag to output a debugging log [4].
- A workaround I have seen is running 'gcloud config set app/use_deprecated_preparation True'. Once that property is set, rerun the deployment. Note that I haven't tested if this works but it worked for some others on this thread in GitHub [5].
2) you are running into some limitation and using 'appcfg.py update [DEPLOYMENTS]' to deploy instead will output a more understandable error message [6][7].
- You may be running into "Exceeded the limit of 1000 files per directory..." Which is a hard limit in Google App Engine [8].
- A simple workaround is to not put a large amount of files in a single directory, or add in an exclude in 'skip_files' in your app.yaml file [9].
I hope one of these suggestions helps to resolve your deployment issue. If you still require more help you could try asking a question with more detail on the steps you took and the issues you encountered in StackOverflow [10].