This error typically means that the tax rate associated with the erring invoice was deleted in Xero.
To check if this is case, you will need to go into your Xero console:
1) Click on Accounting
2) Select Advanced
3) Click Tax Rates
From there, scroll through your list of tax rates to see if the tax rate that was applied to the erring invoice is still there.
If it is not, it means it was deleted at some point. You can try re-add it to your account by clicking New Tax Rate and completing the display and component fields:
Once you have added it back, wait for next sync to place as this should clear the error and allow the erring invoice to sync through to Xero. If it doesn't, you will need to manually 'Ignore Error' in Recurly and then add the invoice to your Xero console.