The error message will typically give details of the cause. Some examples and solutions are:
A Project and Cost Centre code must be specified when the Nominal account has been set up for compulsory costing.
Either enter costing information in the import, or untick 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.