When we deploy project on Google App Engine and due to some error it got cancelled. It may happen if user manually cancel the deployment of project in between or due to slow network or any other reason. This leads to incomplete deployment of projects and revoke deploy from the next time as the existing deployment is in progress. Rollback is the option to undo the previous deployment that failed to complete but marked as in progress deployment.
There are two ways to solve this using rollback option of appcfg.py command.

Solution 1:-

1) Set environment variable for python

2) Place your project’s app.yaml in 

C:\Program Files(x86)\Google\google_appengine

3) Open cmd and using change the directory to 

cd C:\Program Files (x86)\Google\google_appengine

4) Run the command

python appcfg.py rollback app.yaml

5) Remove the app.yaml file from that folder after rollback as the this command uses app.yaml information when you upload the app.