Hi Amelia (accounts1231)
Welcome to the Community forum. This error message usually occurs when the account number is invalid or dosen't exist in your file.
I recommend checking your file has the account number entered in the import file, and it's not missing any information. For example, sometimes when opening a document in Excel, it might display the information incorrectly then that gets saved. e.g. it displays 1-1000 as 11000 and then when trying to import, AccountRight does not recognise the changed account number as a valid number.
If the account numbers are indeed the same, could you send through a screenshot of your account list and the file you are attempting to import so I can assist you further?