This error means that the account record cannot be found, the Exception Log associated with the rejected batch will also provide the reference to the account it is looking for.
To resolve this error:
Click Nominal Ledger
Select Trading Partners
Use the search function and enter the reference provided in the Exceptions Log. Typically you won't find an account and a new account needs to be created with that Account Reference.
However, if it does find one, the account might be flagged as only a Customer or as only a Supplier account. To resolve this:
In the Trading Partners area, click the Details tab.
On the right, in the Controls section, check the fields for both Customer and Supplier. It might be that you are importing to the Supplier account but only the Customer flag is set to Yes.
Setting the Supplier flag to Yes will create a record in the Purchase Ledger.
Setting the Customer flag to Yes will create a record in the Sales Ledger.