This error message occurs when wrong coding is used. 

For instance, if you used income code instead of expenses code on VAT or when you try to match EXPENSE account against REVENUE tax type.

The simplest way to fix this is to change the ledger account in Datamolino. If this doesn't help, please check your Chart of Accounts in Xero and change the definition of the account in Xero. Ledger account and tax code must match.