This error message originates from Twinfield. To resolve it, you need to check the details of the VAT rate of the VAT code used in Twinfield.
Go in Twinfield to Settings / Administration settings / VAT. Double-click the name of the VAT rate to view the details. Check whether a general ledger account has been entered under 'Posting rules'. An empty field causes this error message. To resolve this, enter a general ledger account in the field and save it.