Hi Greg,
In order to add additional datasets, there has to be some sort of unique reference to the Master Table and then you select the additional dataset from the detail table dropdown below. So for example, if Customer is your Master table then your CartInfo table need to have some sort of unique reference such as Full Name of the customer for instance. If the reference is not unique then you will encounter the error you did. We are working on a solution so in the future it wouldn't be so easy to just select any table from the Detail Table list unless it is a valid table with a unique value. This would eliminate the error you encountered.
Can you please provide me the Dropdown field you used on the CartInfo table to reference to Customer? If it is fine with you we can take a look on our end and try to resolve it for you.