Apps with errors cannot be previewed or published. If your app has one or more errors, When you click on Preview or Publish, an error dialog will be shown.

You can identify all errors in App from the tree view on the left. Controls with errors will have an error icon in the tree view. Hovering over the error will show the error description and clicking on the error icon will open the general properties or rule builder ( based on where the error is).
Once all the errors are resolved, the app creator can preview or publish the app.