TaxCloud's system automatically triggers this notice if/ when a merchant has several transactions (to multiple addresses) but all the data is processed through their Test site, while no transactions are processed through their Live site.
This could happen for various reason. For example, a merchant may mistakenly use their Test API credentials, instead of their Live API credentials in their production environment. If this were to happen, TaxCloud faces the risk of inadvertently filing zero returns even though the merchant did, in fact have sales.
It is critical to ensure that all bonafide sales are processed through Live API credentials.