According to the error message, certain variables aren't sufficiently defined.
Could you please check your IAM allocated roles? You should be something close to an "admin." It appears to be a rights-related error.
Under Cloud Build, look at the Logging sections. Whether there isn't anything, you could try editing the trigger with the API to see if it also fails. It might provide further information about the real error.
Identity requirements To update Cloud Build triggers, you'll need the permission cloudbuild.build.create.
You can also try disabling and enabling the Cloud Build API in your project so a new service account is created, or adding the role “App Engine Deployer” to the cloud build service account, then try to deploy again.