Configure shipping rules on your Merchant Center back office is mandatory to start using Google services.
At least, you have to configure one shipping method to validate Merchant Center set up. In order to setup this, please do as following :
1/ Log on your Merchant Center account
Click on "Tools"
Then on "Shipping and returns"
Click on the "+" button to create a new shipping method
More details in Google doc.
Note : We recommend you to configure all the shipping rules you have on your website, to give the right information to Google.