When you hover over the failed icon in the Edit Data Bar in Hubdoc the above error message from Xero may appear. This is because the 'Show in expense claims' box for the category has not been ticked in your Chart of Accounts in Xero.
To do this, go into your 'Settings' in Xero and then select 'Chart of Accounts'. Here you can click into each category and then click the 'Show in expense claims' box.
After, you can refresh your page in Hubdoc and republish the expense.