The most common cause of this is where the VAT number contains spaces, letters, or character such as – or /. We have also seen this message when the 9 digit VAT number has been entered incorrectly. Also the regional settings on the machine need to be set to English United Kingdom as does the date format. 

We have also had cases where HMRC return this error when the VAT account has been closed down. You would need to contact HMRC to resolve this issue if the client has not deregistered for VAT.