When you open a CSV file in Excel some fields are converted to scientific format, for example 1E+08. When the CSV file is then saved the fields are saved as scientific notation which is not recognised by the EC Sales List (ESL) validation process. To overcome this problem you may wish to consider using the template provided. This can be accessed from the bulk upload page.
For more information on CSV files see the Submitting ESL declarations by CSV page.
Please note: The obligation to submit EC Sales Lists will change if the UK leaves the EU without a deal. Further information can be found here: Exporting and importing goods if the UK leaves the EU without a deal.