This occurs because your CSV file is not properly formatted.
The list() statements in the files import_transactions.php and includes/import_transactions.inc populate the $code_id variable.
ST_JOURNAL:
list($reference, $date, $memo, $amt, $code_id, $taxtype, $dim1_ref, $dim2_ref, $person_type_id, $person_id, $comments)
ST_BANKPAYMENT:
list($reference, $date, $memo, $amt, $ignore, $code_id, $taxtype, $dim1_ref, $dim2_ref, $person_type_id, $person_id, $BranchNo)
ST_BANKDEPOSIT:
list($reference, $date, $memo, $ignore, $amt, $code_id, $taxtype, $dim1_ref, $dim2_ref, $person_type_id, $person_id, $BranchNo)
ST_BANKDEPOSIT / ST_BANKPAYMENT:
list($reference, $date, $memo, $amt, $code_id, $taxtype, $dim1_ref, $dim2_ref, $person_type_id, $person_id, $BranchNo)
ST_SALESORDER / ST_SALESINVOICE:
list($customer_id, $branchNo, $reference, $date, $payment_id, $sales_type_name, $dimension_id, $dimension2_id, $item_code, $item_description, $quantity, $unit, $price, $discountpercentage, $freightcost, $delfrom, $deldate, $delto, $deladdress, $contactphone, $email, $custref, $shipvia, $comments, $exrate)