Had you attempted to authenticate to Google via the feed tool and received the following message:
MCA or Manager Accounts are not eligible to house feeds and you must redo your authentication using a unique email address with admin level permissions that is only attached to the particular sub-account you are trying to connect to.
You will need to log into Merchant Center and add a new email address to the sub-account:
Click on the 3-bar icon in the top right corner of your Merchant Center dashboard
Use the blue + icon to begin adding a new user with Admin permissions
Return to the app and retry your authentication