The error message gives details of the cause. Common examples and solutions:
A Project and Cost Centre code must be specified when the Nominal account has been set up for compulsory costing.
Enter costing information in the import, or clear the Compulsory Costing flag in the nominal account.
Nominal Account code is invalid.
Check that the nominal account exists, and is not hidden.
Please indicate whether the line is a debit or a credit.
Look for 0 or blank debit and credit lines.