This error message means that this account code is not allowed to be published as an expense in Xero.
To resolve it please go into the 'Settings' menu in Xero, select Chart of Accounts, and allow this as an expense code.
Then back in Dext Prepare, navigate to the Account Settings menu and open the Maintain lists tab. Here 'Reload all lists' (from the top right corner) to update the chart of accounts in Dext Prepare. Then you should be able to publish your item.