A common error is including your customer's country code in the VAT registration number field. It should only be entered in the country code column and not be included as part of the customer's VAT number. You can check the format of your customer's VAT registration number on the EU country codes and customer VAT number guidance at EU Country Codes and customer number formats
Please note: If the UK leaves the EU with no deal, the rules on EC Sales lists will change. Further information can be found in VAT for businesses in a no-deal Brexit.