This is the Google App Engine mailing list. To use the PaaS system, you only need to create a project (which you have already done) then go to
console.cloud.google.com/appengine to create your App Engine app. There's no need to create credentials or use a "Google Cloud API" unless you know you need to do so. I suggest installing the Google Cloud SDK (
cloud.google.com/sdk) which provides tools to simplify creating & deploying your app. To learn more about App Engine, please see
cloud.google.com/appengine for its documentation. If you have any specific questions, feel free to ask here but paste any specific commands or screenshots showing where you may have an issue.