If you are using Google Wallet Objects API, make sure that you share your Account in the Merchant Account with the Service Account Email Address (the one you got when you created your credentials - you can get this one from the cloud console https://cloud.google.com/console, the one that ends with @developer.gserviceaccount.com)
Login to the Merchant Dashboard, click on Account Management and then click on Share. Add the email to the list of users.
You'll also want to share your merchant account with any users you want to give preview access to your classes and discoverables.