Invoice not sent to Xero, please try again later.

This is a message fed back from Xero which means the invoice creation process failed.  The most common reason is due to attempting to resend an order that is already sent to Xero.

Check what the notes on the order show.  In this case it shows that the order had already been sent at the time of checkout so it can't be sent again to Xero with the same invoice no and hence is rejected by Xero.  Xeroom sets the invoice no to use per the settings or will use the Xero default setting.  Xeroom sets a flag to prevent duplicate invoices from being created in Xero.  If you definitely want to resend it to create a duplicate (or in the instance that the original order did not appear for any reason) then use the Resend button.  This will add a suffix A/B/C etc for each resend so it obvious to your Xero users and any duplicates can be easily identified.

The custom fields section of the order in WooCommerce will show the invoice sent no and suffix.