When you use OAuth 2.0 for authorization, your app requests authorizations for one or more scopes of access from a Google Account. Google displays a consent screen to the user that includes a summary of your project and its policies and the requested scopes of access. To create the consent screen, you need to go to https://console.cloud.google.com/apis/credentials/consent
When creating the consent screen, the scope should be the DialogFlow API. Having the app in Testing status allows you to start using the app. However, in this case, you'll need to add people as test users. Otherwise, the app should be verified by Google and published.