This app is running in disconnected mode. GraphQL requires connected mode, headless connected mode, or integrated mode to work.

Libraries such as graphql-tools can provide GraphQL API mocking capabilities, which could enable disconnected GraphQL. This is not supported out of the box, however.

To view the GraphQL samples, restart the app using jss start:connected  or deploy the app to Sitecore to run in integrated mode per the JSS documentation.