fEach shop needs Master Fields correctly mapped, as they are often later used in channels.
If you don't have the Master Fields mapped, this simply means your shop is not correctly added or configured.
Reason: Shop Creation not finished
If you don't finish configuring your shop by creating Master Fields, you may end up with the Attention required error. This simply means you need to finish the configuration.
To do this, go to Master Fields and create rules for all the required fields in your shop.
Reason: Fields downloaded from your shop changed
You did create Master Fields already, but do you see this error anyway? Some field from your shop you used in Master Fields changed or is unavailable anymore.
To solve this problem, go to your shop's Master Fields, and create rules for the required fields that are marked red and do not have any rules.