Before connecting your Google Express account you should create a channel. Since Google Express requires a storefront it should be added as a subchannel to your store.
Go to Menu > Settings > Channels
>> Click add a channel on the upper right-hand corner
>> Select Google Express
>> Choose the country
>> Name your channel
>> Click the Create button
>> Enter your Google Merchant Id
>> Click the Save button
*** Please add the following user name to your user list and give all available access rights to StoreAutomator at Google Merchant Center.